RFC 3931 L2TPv3 March 2005
contain any pseudowire-type specific details that are outside the
scope of this base specification.
When the designation between L2TPv2 and L2TPv3 is necessary, L2TP as
defined in RFC 2661 will be referred to as "L2TPv2", corresponding to
the value in the Version field of an L2TP header. (Layer 2
Forwarding, L2F, [RFC2341] was defined as "version 1".) At times,
L2TP as defined in this document will be referred to as "L2TPv3".
Otherwise, the acronym "L2TP" will refer to L2TPv3 or L2TP in
general.
1.1. Changes from RFC 2661
Many of the protocol constructs described in this document are
carried over from RFC 2661. Changes include clarifications based on
years of interoperability and deployment experience as well as
modifications to either improve protocol operation or provide a
clearer separation from PPP. The intent of these modifications is to
achieve a healthy balance between code reuse, interoperability
experience, and a directed evolution of L2TP as it is applied to new
tasks.
Notable differences between L2TPv2 and L2TPv3 include the following:
Separation of all PPP-related AVPs, references, etc., including a
portion of the L2TP data header that was specific to the needs of
PPP. The PPP-specific constructs are described in a companion
document.
Transition from a 16-bit Session ID and Tunnel ID to a 32-bit
Session ID and Control Connection ID, respectively.
Extension of the Tunnel Authentication mechanism to cover the
entire control message rather than just a portion of certain
messages.
Details of these changes and a recommendation for transitioning to
L2TPv3 are discussed in Section 4.7.
1.2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Lau, et al. Standards Track [Page 4]

RFC 3931 L2TPv3 March 20051.3. Terminology
Attribute Value Pair (AVP)
The variable-length concatenation of a unique Attribute
(represented by an integer), a length field, and a Value
containing the actual value identified by the attribute. Zero or
more AVPs make up the body of control messages, which are used in
the establishment, maintenance, and teardown of control
connections. This basic construct is sometimes referred to as a
Type-Length-Value (TLV) in some specifications. (See also:
Control Connection, Control Message.)
Call (Circuit Up)
The action of transitioning a circuit on an L2TP Access
Concentrator (LAC) to an "up" or "active" state. A call may be
dynamically established through signaling properties (e.g., an
incoming or outgoing call through the Public Switched Telephone
Network (PSTN)) or statically configured (e.g., provisioning a
Virtual Circuit on an interface). A call is defined by its
properties (e.g., type of call, called number, etc.) and its data
traffic. (See also: Circuit, Session, Incoming Call, Outgoing
Call, Outgoing Call Request.)
Circuit
A general term identifying any one of a wide range of L2
connections. A circuit may be virtual in nature (e.g., an ATM
PVC, an IEEE 802 VLAN, or an L2TP session), or it may have direct
correlation to a physical layer (e.g., an RS-232 serial line).
Circuits may be statically configured with a relatively long-lived
uptime, or dynamically established with signaling to govern the
establishment, maintenance, and teardown of the circuit. For the
purposes of this document, a statically configured circuit is
considered to be essentially the same as a very simple, long-
lived, dynamic circuit. (See also: Call, Remote System.)
Client
(See Remote System.)
Control Connection
An L2TP control connection is a reliable control channel that is
used to establish, maintain, and release individual L2TP sessions
as well as the control connection itself. (See also: Control
Message, Data Channel.)
Lau, et al. Standards Track [Page 5]

RFC 3931 L2TPv3 March 2005
Control Message
An L2TP message used by the control connection. (See also:
Control Connection.)
Data Message
Message used by the data channel. (a.k.a. Data Packet, See also:
Data Channel.)
Data Channel
The channel for L2TP-encapsulated data traffic that passes between
two LCCEs over a Packet-Switched Network (i.e., IP). (See also:
Control Connection, Data Message.)
Incoming Call
The action of receiving a call (circuit up event) on an LAC. The
call may have been placed by a remote system (e.g., a phone call
over a PSTN), or it may have been triggered by a local event
(e.g., interesting traffic routed to a virtual interface). An
incoming call that needs to be tunneled (as determined by the LAC)
results in the generation of an L2TP ICRQ message. (See also:
Call, Outgoing Call, Outgoing Call Request.)
L2TP Access Concentrator (LAC)
If an L2TP Control Connection Endpoint (LCCE) is being used to
cross-connect an L2TP session directly to a data link, we refer to
it as an L2TP Access Concentrator (LAC). An LCCE may act as both
an L2TP Network Server (LNS) for some sessions and an LAC for
others, so these terms must only be used within the context of a
given set of sessions unless the LCCE is in fact single purpose
for a given topology. (See also: LCCE, LNS.)
L2TP Control Connection Endpoint (LCCE)
An L2TP node that exists at either end of an L2TP control
connection. May also be referred to as an LAC or LNS, depending
on whether tunneled frames are processed at the data link (LAC) or
network layer (LNS). (See also: LAC, LNS.)
L2TP Network Server (LNS)
If a given L2TP session is terminated at the L2TP node and the
encapsulated network layer (L3) packet processed on a virtual
interface, we refer to this L2TP node as an L2TP Network Server
Lau, et al. Standards Track [Page 6]

RFC 3931 L2TPv3 March 2005
(LNS). A given LCCE may act as both an LNS for some sessions and
an LAC for others, so these terms must only be used within the
context of a given set of sessions unless the LCCE is in fact
single purpose for a given topology. (See also: LCCE, LAC.)
Outgoing Call
The action of placing a call by an LAC, typically in response to
policy directed by the peer in an Outgoing Call Request. (See
also: Call, Incoming Call, Outgoing Call Request.)
Outgoing Call Request
A request sent to an LAC to place an outgoing call. The request
contains specific information not known a priori by the LAC (e.g.,
a number to dial). (See also: Call, Incoming Call, Outgoing
Call.)
Packet-Switched Network (PSN)
A network that uses packet switching technology for data delivery.
For L2TPv3, this layer is principally IP. Other examples include
MPLS, Frame Relay, and ATM.
Peer
When used in context with L2TP, Peer refers to the far end of an
L2TP control connection (i.e., the remote LCCE). An LAC's peer
may be either an LNS or another LAC. Similarly, an LNS's peer may
be either an LAC or another LNS. (See also: LAC, LCCE, LNS.)
Pseudowire (PW)
An emulated circuit as it traverses a PSN. There is one
Pseudowire per L2TP Session. (See also: Packet-Switched Network,
Session.)
Pseudowire Type
The payload type being carried within an L2TP session. Examples
include PPP, Ethernet, and Frame Relay. (See also: Session.)
Remote System
An end system or router connected by a circuit to an LAC.
Lau, et al. Standards Track [Page 7]

RFC 3931 L2TPv3 March 2005
Session
An L2TP session is the entity that is created between two LCCEs in
order to exchange parameters for and maintain an emulated L2
connection. Multiple sessions may be associated with a single
Control Connection.
Zero-Length Body (ZLB) Message
A control message with only an L2TP header. ZLB messages are used
only to acknowledge messages on the L2TP reliable control
connection. (See also: Control Message.)
2. Topology
L2TP operates between two L2TP Control Connection Endpoints (LCCEs),
tunneling traffic across a packet network. There are three
predominant tunneling models in which L2TP operates: LAC-LNS (or vice
versa), LAC-LAC, and LNS-LNS. These models are diagrammed below.
(Dotted lines designate network connections. Solid lines designate
circuit connections.)
Figure 2.0: L2TP Reference Models
(a) LAC-LNS Reference Model: On one side, the LAC receives traffic
from an L2 circuit, which it forwards via L2TP across an IP or other
packet-based network. On the other side, an LNS logically terminates
the L2 circuit locally and routes network traffic to the home
network. The action of session establishment is driven by the LAC
(as an incoming call) or the LNS (as an outgoing call).
+-----+ L2 +-----+ +-----+
| |------| LAC |.........[ IP ].........| LNS |...[home network]
+-----+ +-----+ +-----+
remote
system
|<-- emulated service -->|
|<----------- L2 service ------------>|
(b) LAC-LAC Reference Model: In this model, both LCCEs are LACs.
Each LAC forwards circuit traffic from the remote system to the peer
LAC using L2TP, and vice versa. In its simplest form, an LAC acts as
a simple cross-connect between a circuit to a remote system and an
L2TP session. This model typically involves symmetric establishment;
that is, either side of the connection may initiate a session at any
time (or simultaneously, in which a tie breaking mechanism is
utilized).
Lau, et al. Standards Track [Page 8]

RFC 3931 L2TPv3 March 2005
+-----+ L2 +-----+ +-----+ L2 +-----+
| |------| LAC |........[ IP ]........| LAC |------| |
+-----+ +-----+ +-----+ +-----+
remote remote
system system
|<- emulated service ->|
|<----------------- L2 service ----------------->|
(c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs. A
user-level, traffic-generated, or signaled event typically drives
session establishment from one side of the tunnel. For example, a
tunnel generated from a PC by a user, or automatically by customer
premises equipment.
+-----+ +-----+
[home network]...| LNS |........[ IP ]........| LNS |...[home network]
+-----+ +-----+
|<- emulated service ->|
|<---- L2 service ---->|
Note: In L2TPv2, user-driven tunneling of this type is often referred
to as "voluntary tunneling" [RFC2809]. Further, an LNS acting as
part of a software package on a host is sometimes referred to as an
"LAC Client" [RFC2661].
3. Protocol Overview
L2TP is comprised of two types of messages, control messages and data
messages (sometimes referred to as "control packets" and "data
packets", respectively). Control messages are used in the
establishment, maintenance, and clearing of control connections and
sessions. These messages utilize a reliable control channel within
L2TP to guarantee delivery (see Section 4.2 for details). Data
messages are used to encapsulate the L2 traffic being carried over
the L2TP session. Unlike control messages, data messages are not
retransmitted when packet loss occurs.
The L2TPv3 control message format defined in this document borrows
largely from L2TPv2. These control messages are used in conjunction
with the associated protocol state machines that govern the dynamic
setup, maintenance, and teardown for L2TP sessions. The data message
format for tunneling data packets may be utilized with or without the
L2TP control channel, either via manual configuration or via other
signaling methods to pre-configure or distribute L2TP session
information. Utilization of the L2TP data message format with other
signaling methods is outside the scope of this document.
Lau, et al. Standards Track [Page 9]

RFC 3931 L2TPv3 March 2005
Figure 3.0: L2TPv3 Structure
+-------------------+ +-----------------------+
| Tunneled Frame | | L2TP Control Message |
+-------------------+ +-----------------------+
| L2TP Data Header | | L2TP Control Header |
+-------------------+ +-----------------------+
| L2TP Data Channel | | L2TP Control Channel |
| (unreliable) | | (reliable) |
+-------------------+----+-----------------------+
| Packet-Switched Network (IP, FR, MPLS, etc.) |
+------------------------------------------------+
Figure 3.0 depicts the relationship of control messages and data
messages over the L2TP control and data channels, respectively. Data
messages are passed over an unreliable data channel, encapsulated by
an L2TP header, and sent over a Packet-Switched Network (PSN) such as
IP, UDP, Frame Relay, ATM, MPLS, etc. Control messages are sent over
a reliable L2TP control channel, which operates over the same PSN.
The necessary setup for tunneling a session with L2TP consists of two
steps: (1) Establishing the control connection, and (2) establishing
a session as triggered by an incoming call or outgoing call. An L2TP
session MUST be established before L2TP can begin to forward session
frames. Multiple sessions may be bound to a single control
connection, and multiple control connections may exist between the
same two LCCEs.
3.1. Control Message Types
The Message Type AVP (see Section 5.4.1) defines the specific type of
control message being sent.
This document defines the following control message types (see
Sections 6.1 through 6.15 for details on the construction and use of
each message):
Control Connection Management
0 (reserved)
1 (SCCRQ) Start-Control-Connection-Request
2 (SCCRP) Start-Control-Connection-Reply
3 (SCCCN) Start-Control-Connection-Connected
4 (StopCCN) Stop-Control-Connection-Notification
5 (reserved)
6 (HELLO) Hello
20 (ACK) Explicit Acknowledgement
Lau, et al. Standards Track [Page 10]

RFC 3931 L2TPv3 March 2005
The L and S bits MUST be set to 1, indicating that the Length field
and sequence numbers are present.
The x bits are reserved for future extensions. All reserved bits
MUST be set to 0 on outgoing messages and ignored on incoming
messages.
The Ver field indicates the version of the L2TP control message
header described in this document. On sending, this field MUST be
set to 3 for all messages (unless operating in an environment that
includes L2TPv2 [RFC2661] and/or L2F [RFC2341] as well, see Section4.1 for details).
The Length field indicates the total length of the message in octets,
always calculated from the start of the control message header itself
(beginning with the T bit).
The Control Connection ID field contains the identifier for the
control connection. L2TP control connections are named by
identifiers that have local significance only. That is, the same
control connection will be given unique Control Connection IDs by
each LCCE from within each endpoint's own Control Connection ID
number space. As such, the Control Connection ID in each message is
that of the intended recipient, not the sender. Non-zero Control
Connection IDs are selected and exchanged as Assigned Control
Connection ID AVPs during the creation of a control connection.
Ns indicates the sequence number for this control message, beginning
at zero and incrementing by one (modulo 2**16) for each message sent.
See Section 4.2 for more information on using this field.
Nr indicates the sequence number expected in the next control message
to be received. Thus, Nr is set to the Ns of the last in-order
message received plus one (modulo 2**16). See Section 4.2 for more
information on using this field.
3.2.2. L2TP Data Message
In general, an L2TP data message consists of a (1) Session Header,
(2) an optional L2-Specific Sublayer, and (3) the Tunnel Payload, as
depicted below.
Lau, et al. Standards Track [Page 12]

RFC 3931 L2TPv3 March 2005
Figure 3.2.2: L2TP Data Message Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Specific Sublayer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Payload ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The L2TP Session Header is specific to the encapsulating PSN over
which the L2TP traffic is delivered. The Session Header MUST provide
(1) a method of distinguishing traffic among multiple L2TP data
sessions and (2) a method of distinguishing data messages from
control messages.
Each type of encapsulating PSN MUST define its own session header,
clearly identifying the format of the header and parameters necessary
to setup the session. Section 4.1 defines two session headers, one
for transport over UDP and one for transport over IP.
The L2-Specific Sublayer is an intermediary layer between the L2TP
session header and the start of the tunneled frame. It contains
control fields that are used to facilitate the tunneling of each
frame (e.g., sequence numbers or flags). The Default L2-Specific
Sublayer for L2TPv3 is defined in Section 4.6.
The Data Message Header is followed by the Tunnel Payload, including
any necessary L2 framing as defined in the payload-specific companion
documents.
3.3. Control Connection Management
The L2TP control connection handles dynamic establishment, teardown,
and maintenance of the L2TP sessions and of the control connection
itself. The reliable delivery of control messages is described in
Section 4.2.
This section describes typical control connection establishment and
teardown exchanges. It is important to note that, in the diagrams
that follow, the reliable control message delivery mechanism exists
independently of the L2TP state machine. For instance, Explicit
Acknowledgement (ACK) messages may be sent after any of the control
messages indicated in the exchanges below if an acknowledgment is not
piggybacked on a later control message.
Lau, et al. Standards Track [Page 13]

RFC 3931 L2TPv3 March 2005
LCCEs are identified during control connection establishment either
by the Host Name AVP, the Router ID AVP, or a combination of the two
(see Section 5.4.3). The identity of a peer LCCE is central to
selecting proper configuration parameters (i.e., Hello interval,
window size, etc.) for a control connection, as well as for
determining how to set up associated sessions within the control
connection, password lookup for control connection authentication,
control connection level tie breaking, etc.
3.3.1. Control Connection Establishment
Establishment of the control connection involves an exchange of AVPs
that identifies the peer and its capabilities.
A three-message exchange is used to establish the control connection.
The following is a typical message exchange:
LCCE A LCCE B
------ ------
SCCRQ ->
<- SCCRP
SCCCN ->
3.3.2. Control Connection Teardown
Control connection teardown may be initiated by either LCCE and is
accomplished by sending a single StopCCN control message. As part of
the reliable control message delivery mechanism, the recipient of a
StopCCN MUST send an ACK message to acknowledge receipt of the
message and maintain enough control connection state to properly
accept StopCCN retransmissions over at least a full retransmission
cycle (in case the ACK message is lost). The recommended time for a
full retransmission cycle is at least 31 seconds (see Section 4.2).
The following is an example of a typical control message exchange:
LCCE A LCCE B
------ ------
StopCCN ->
(Clean up)
(Wait)
(Clean up)
An implementation may shut down an entire control connection and all
sessions associated with the control connection by sending the
StopCCN. Thus, it is not necessary to clear each session
individually when tearing down the whole control connection.
Lau, et al. Standards Track [Page 14]

RFC 3931 L2TPv3 March 20053.4. Session Management
After successful control connection establishment, individual
sessions may be created. Each session corresponds to a single data
stream between the two LCCEs. This section describes the typical
call establishment and teardown exchanges.
3.4.1. Session Establishment for an Incoming Call
A three-message exchange is used to establish the session. The
following is a typical sequence of events:
LCCE A LCCE B
------ ------
(Call
Detected)
ICRQ ->
<- ICRP
(Call
Accepted)
ICCN ->
3.4.2. Session Establishment for an Outgoing Call
A three-message exchange is used to set up the session. The
following is a typical sequence of events:
LCCE A LCCE B
------ ------
<- OCRQ
OCRP ->
(Perform
Call
Operation)
OCCN ->
(Call Operation
Completed
Successfully)
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RFC 3931 L2TPv3 March 20053.4.3. Session Teardown
Session teardown may be initiated by either the LAC or LNS and is
accomplished by sending a CDN control message. After the last
session is cleared, the control connection MAY be torn down as well
(and typically is). The following is an example of a typical control
message exchange:
LCCE A LCCE B
------ ------
CDN ->
(Clean up)
(Clean up)
4. Protocol Operation4.1. L2TP Over Specific Packet-Switched Networks (PSNs)
L2TP may operate over a variety of PSNs. There are two modes
described for operation over IP, L2TP directly over IP (see Section4.1.1) and L2TP over UDP (see Section 4.1.2). L2TPv3 implementations
MUST support L2TP over IP and SHOULD support L2TP over UDP for better
NAT and firewall traversal, and for easier migration from L2TPv2.
L2TP over other PSNs may be defined, but the specifics are outside
the scope of this document. Examples of L2TPv2 over other PSNs
include [RFC3070] and [RFC3355].
The following field definitions are defined for use in all L2TP
Session Header encapsulations.
Session ID
A 32-bit field containing a non-zero identifier for a session.
L2TP sessions are named by identifiers that have local
significance only. That is, the same logical session will be
given different Session IDs by each end of the control connection
for the life of the session. When the L2TP control connection is
used for session establishment, Session IDs are selected and
exchanged as Local Session ID AVPs during the creation of a
session. The Session ID alone provides the necessary context for
all further packet processing, including the presence, size, and
value of the Cookie, the type of L2-Specific Sublayer, and the
type of payload being tunneled.
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RFC 3931 L2TPv3 March 2005
Cookie
The optional Cookie field contains a variable-length value
(maximum 64 bits) used to check the association of a received data
message with the session identified by the Session ID. The Cookie
MUST be set to the configured or signaled random value for this
session. The Cookie provides an additional level of guarantee
that a data message has been directed to the proper session by the
Session ID. A well-chosen Cookie may prevent inadvertent
misdirection of stray packets with recently reused Session IDs,
Session IDs subject to packet corruption, etc. The Cookie may
also provide protection against some specific malicious packet
insertion attacks, as described in Section 8.2.
When the L2TP control connection is used for session
establishment, random Cookie values are selected and exchanged as
Assigned Cookie AVPs during session creation.
4.1.1. L2TPv3 over IP
L2TPv3 over IP (both versions) utilizes the IANA-assigned IP protocol
ID 115.
4.1.1.1. L2TPv3 Session Header Over IP
Unlike L2TP over UDP, the L2TPv3 session header over IP is free of
any restrictions imposed by coexistence with L2TPv2 and L2F. As
such, the header format has been designed to optimize packet
processing. The following session header format is utilized when
operating L2TPv3 over IP:
Figure 4.1.1.1: L2TPv3 Session Header Over IP
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie (optional, maximum 64 bits)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Session ID and Cookie fields are as defined in Section 4.1. The
Session ID of zero is reserved for use by L2TP control messages (see
Section 4.1.1.2).
Lau, et al. Standards Track [Page 17]

RFC 3931 L2TPv3 March 20054.1.1.2. L2TP Control and Data Traffic over IP
Unlike L2TP over UDP, which uses the T bit to distinguish between
L2TP control and data packets, L2TP over IP uses the reserved Session
ID of zero (0) when sending control messages. It is presumed that
checking for the zero Session ID is more efficient -- both in header
size for data packets and in processing speed for distinguishing
between control and data messages -- than checking a single bit.
The entire control message header over IP, including the zero session
ID, appears as follows:
Figure 4.1.1.2: L2TPv3 Control Message Header Over IP
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (32 bits of zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|L|x|x|S|x|x|x|x|x|x|x| Ver | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns | Nr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Named fields are as defined in Section 3.2.1. Note that the Length
field is still calculated from the beginning of the control message
header, beginning with the T bit. It does NOT include the "(32 bits
of zeros)" depicted above.
When operating directly over IP, L2TP packets lose the ability to
take advantage of the UDP checksum as a simple packet integrity
check, which is of particular concern for L2TP control messages.
Control Message Authentication (see Section 4.3), even with an empty
password field, provides for a sufficient packet integrity check and
SHOULD always be enabled.
4.1.2. L2TP over UDP
L2TPv3 over UDP must consider other L2 tunneling protocols that may
be operating in the same environment, including L2TPv2 [RFC2661] and
L2F [RFC2341].
While there are efficiencies gained by running L2TP directly over IP,
there are possible side effects as well. For instance, L2TP over IP
is not as NAT-friendly as L2TP over UDP.
Lau, et al. Standards Track [Page 18]

RFC 3931 L2TPv3 March 20054.1.2.1. L2TP Session Header Over UDP
The following session header format is utilized when operating L2TPv3
over UDP:
Figure 4.1.2.1: L2TPv3 Session Header over UDP
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|x|x|x|x|x|x|x|x|x|x|x| Ver | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie (optional, maximum 64 bits)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The T bit MUST be set to 0, indicating that this is a data message.
The x bits and Reserved field are reserved for future extensions.
All reserved values MUST be set to 0 on outgoing messages and ignored
on incoming messages.
The Ver field MUST be set to 3, indicating an L2TPv3 message.
Note that the initial bits 1, 4, 6, and 7 have meaning in L2TPv2
[RFC2661], and are deprecated and marked as reserved in L2TPv3.
Thus, for UDP mode on a system that supports both versions of L2TP,
it is important that the Ver field be inspected first to determine
the Version of the header before acting upon any of these bits.
The Session ID and Cookie fields are as defined in Section 4.1.
4.1.2.2. UDP Port Selection
The method for UDP Port Selection defined in this section is
identical to that defined for L2TPv2 [RFC2661].
When negotiating a control connection over UDP, control messages MUST
be sent as UDP datagrams using the registered UDP port 1701
[RFC1700]. The initiator of an L2TP control connection picks an
available source UDP port (which may or may not be 1701) and sends to
the desired destination address at port 1701. The recipient picks a
free port on its own system (which may or may not be 1701) and sends
its reply to the initiator's UDP port and address, setting its own
source port to the free port it found.
Lau, et al. Standards Track [Page 19]

RFC 3931 L2TPv3 March 2005
Any subsequent traffic associated with this control connection
(either control traffic or data traffic from a session established
through this control connection) must use these same UDP ports.
It has been suggested that having the recipient choose an arbitrary
source port (as opposed to using the destination port in the packet
initiating the control connection, i.e., 1701) may make it more
difficult for L2TP to traverse some NAT devices. Implementations
should consider the potential implication of this capability before
choosing an arbitrary source port. A NAT device that can pass TFTP
traffic with variant UDP ports should be able to pass L2TP UDP
traffic since both protocols employ similar policies with regard to
UDP port selection.
4.1.2.3. UDP Checksum
The tunneled frames that L2TP carry often have their own checksums or
integrity checks, rendering the UDP checksum redundant for much of
the L2TP data message contents. Thus, UDP checksums MAY be disabled
in order to reduce the associated packet processing burden at the
L2TP endpoints.
The L2TP header itself does not have its own checksum or integrity
check. However, use of the L2TP Session ID and Cookie pair guards
against accepting an L2TP data message if corruption of the Session
ID or associated Cookie has occurred. When the L2-Specific Sublayer
is present in the L2TP header, there is no built-in integrity check
for the information contained therein if UDP checksums or some other
integrity check is not employed. IPsec (see Section 4.1.3) may be
used for strong integrity protection of the entire contents of L2TP
data messages.
UDP checksums MUST be enabled for L2TP control messages.
4.1.3. L2TP and IPsec
The L2TP data channel does not provide cryptographic security of any
kind. If the L2TP data channel operates over a public or untrusted
IP network where privacy of the L2TP data is of concern or
sophisticated attacks against L2TP are expected to occur, IPsec
[RFC2401] MUST be made available to secure the L2TP traffic.
Either L2TP over UDP or L2TP over IP may be secured with IPsec.
[RFC3193] defines the recommended method for securing L2TPv2. L2TPv3
possesses identical characteristics to IPsec as L2TPv2 when running
over UDP and implementations MUST follow the same recommendation.
When operating over IP directly, [RFC3193] still applies, though
references to UDP source and destination ports (in particular, those
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RFC 3931 L2TPv3 March 2005
in Section 4, "IPsec Filtering details when protecting L2TP") may be
ignored. Instead, the selectors used to identify L2TPv3 traffic are
simply the source and destination IP addresses for the tunnel
endpoints together with the L2TPv3 IP protocol type, 115.
In addition to IP transport security, IPsec defines a mode of
operation that allows tunneling of IP packets. The packet-level
encryption and authentication provided by IPsec tunnel mode and that
provided by L2TP secured with IPsec provide an equivalent level of
security for these requirements.
IPsec also defines access control features that are required of a
compliant IPsec implementation. These features allow filtering of
packets based upon network and transport layer characteristics such
as IP address, ports, etc. In the L2TP tunneling model, analogous
filtering may be performed at the network layer above L2TP. These
network layer access control features may be handled at an LCCE via
vendor-specific authorization features, or at the network layer
itself by using IPsec transport mode end-to-end between the
communicating hosts. The requirements for access control mechanisms
are not a part of the L2TP specification, and as such, are outside
the scope of this document.
Protecting the L2TP packet stream with IPsec does, in turn, also
protect the data within the tunneled session packets while
transported from one LCCE to the other. Such protection must not be
considered a substitution for end-to-end security between
communicating hosts or applications.
4.1.4. IP Fragmentation Issues
Fragmentation and reassembly in network equipment generally require
significantly greater resources than sending or receiving a packet as
a single unit. As such, fragmentation and reassembly should be
avoided whenever possible. Ideal solutions for avoiding
fragmentation include proper configuration and management of MTU
sizes among the Remote System, the LCCE, and the IP network, as well
as adaptive measures that operate with the originating host (e.g.,
[RFC1191], [RFC1981]) to reduce the packet sizes at the source.
An LCCE MAY fragment a packet before encapsulating it in L2TP. For
example, if an IPv4 packet arrives at an LCCE from a Remote System
that, after encapsulation with its associated framing, L2TP, and IP,
does not fit in the available path MTU towards its LCCE peer, the
local LCCE may perform IPv4 fragmentation on the packet before tunnel
encapsulation. This creates two (or more) L2TP packets, each
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carrying an IPv4 fragment with its associated framing. This
ultimately has the effect of placing the burden of fragmentation on
the LCCE, while reassembly occurs on the IPv4 destination host.
If an IPv6 packet arrives at an LCCE from a Remote System that, after
encapsulation with associated framing, L2TP and IP, does not fit in
the available path MTU towards its L2TP peer, the Generic Packet
Tunneling specification [RFC2473], Section 7.1 SHOULD be followed.
In this case, the LCCE should either send an ICMP Packet Too Big
message to the data source, or fragment the resultant L2TP/IP packet
(for reassembly by the L2TP peer).
If the amount of traffic requiring fragmentation and reassembly is
rather light, or there are sufficiently optimized mechanisms at the
tunnel endpoints, fragmentation of the L2TP/IP packet may be
sufficient for accommodating mismatched MTUs that cannot be managed
by more efficient means. This method effectively emulates a larger
MTU between tunnel endpoints and should work for any type of L2-
encapsulated packet. Note that IPv6 does not support "in-flight"
fragmentation of data packets. Thus, unlike IPv4, the MTU of the
path towards an L2TP peer must be known in advance (or the last
resort IPv6 minimum MTU of 1280 bytes utilized) so that IPv6
fragmentation may occur at the LCCE.
In summary, attempting to control the source MTU by communicating
with the originating host, forcing that an MTU be sufficiently large
on the path between LCCE peers to tunnel a frame from any other
interface without fragmentation, fragmenting IP packets before
encapsulation with L2TP/IP, or fragmenting the resultant L2TP/IP
packet between the tunnel endpoints, are all valid methods for
managing MTU mismatches. Some are clearly better than others
depending on the given deployment. For example, a passive monitoring
application using L2TP would certainly not wish to have ICMP messages
sent to a traffic source. Further, if the links connecting a set of
LCCEs have a very large MTU (e.g., SDH/SONET) and it is known that
the MTU of all links being tunneled by L2TP have smaller MTUs (e.g.,
1500 bytes), then any IP fragmentation and reassembly enabled on the
participating LCCEs would never be utilized. An implementation MUST
implement at least one of the methods described in this section for
managing mismatched MTUs, based on careful consideration of how the
final product will be deployed.
L2TP-specific fragmentation and reassembly methods, which may or may
not depend on the characteristics of the type of link being tunneled
(e.g., judicious packing of ATM cells), may be defined as well, but
these methods are outside the scope of this document.
Lau, et al. Standards Track [Page 22]

RFC 3931 L2TPv3 March 20054.2. Reliable Delivery of Control Messages
L2TP provides a lower level reliable delivery service for all control
messages. The Nr and Ns fields of the control message header (see
Section 3.2.1) belong to this delivery mechanism. The upper level
functions of L2TP are not concerned with retransmission or ordering
of control messages. The reliable control messaging mechanism is a
sliding window mechanism that provides control message retransmission
and congestion control. Each peer maintains separate sequence number
state for each control connection.
The message sequence number, Ns, begins at 0. Each subsequent
message is sent with the next increment of the sequence number. The
sequence number is thus a free-running counter represented modulo
65536. The sequence number in the header of a received message is
considered less than or equal to the last received number if its
value lies in the range of the last received number and the preceding
32767 values, inclusive. For example, if the last received sequence
number was 15, then messages with sequence numbers 0 through 15, as
well as 32784 through 65535, would be considered less than or equal.
Such a message would be considered a duplicate of a message already
received and ignored from processing. However, in order to ensure
that all messages are acknowledged properly (particularly in the case
of a lost ACK message), receipt of duplicate messages MUST be
acknowledged by the reliable delivery mechanism. This acknowledgment
may either piggybacked on a message in queue or sent explicitly via
an ACK message.
All control messages take up one slot in the control message sequence
number space, except the ACK message. Thus, Ns is not incremented
after an ACK message is sent.
The last received message number, Nr, is used to acknowledge messages
received by an L2TP peer. It contains the sequence number of the
message the peer expects to receive next (e.g., the last Ns of a
non-ACK message received plus 1, modulo 65536). While the Nr in a
received ACK message is used to flush messages from the local
retransmit queue (see below), the Nr of the next message sent is not
updated by the Ns of the ACK message. Nr SHOULD be sanity-checked
before flushing the retransmit queue. For instance, if the Nr
received in a control message is greater than the last Ns sent plus 1
modulo 65536, the control message is clearly invalid.
The reliable delivery mechanism at a receiving peer is responsible
for making sure that control messages are delivered in order and
without duplication to the upper level. Messages arriving out-of-
order may be queued for in-order delivery when the missing messages
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RFC 3931 L2TPv3 March 2005
are received. Alternatively, they may be discarded, thus requiring a
retransmission by the peer. When dropping out-of-order control
packets, Nr MAY be updated before the packet is discarded.
Each control connection maintains a queue of control messages to be
transmitted to its peer. The message at the front of the queue is
sent with a given Ns value and is held until a control message
arrives from the peer in which the Nr field indicates receipt of this
message. After a period of time (a recommended default is 1 second
but SHOULD be configurable) passes without acknowledgment, the
message is retransmitted. The retransmitted message contains the
same Ns value, but the Nr value MUST be updated with the sequence
number of the next expected message.
Each subsequent retransmission of a message MUST employ an
exponential backoff interval. Thus, if the first retransmission
occurred after 1 second, the next retransmission should occur after 2
seconds has elapsed, then 4 seconds, etc. An implementation MAY
place a cap upon the maximum interval between retransmissions. This
cap SHOULD be no less than 8 seconds per retransmission. If no peer
response is detected after several retransmissions (a recommended
default is 10, but MUST be configurable), the control connection and
all associated sessions MUST be cleared. As it is the first message
to establish a control connection, the SCCRQ MAY employ a different
retransmission maximum than other control messages in order to help
facilitate failover to alternate LCCEs in a timely fashion.
When a control connection is being shut down for reasons other than
loss of connectivity, the state and reliable delivery mechanisms MUST
be maintained and operated for the full retransmission interval after
the final message StopCCN message has been sent (e.g., 1 + 2 + 4 + 8
+ 8... seconds), or until the StopCCN message itself has been
acknowledged.
A sliding window mechanism is used for control message transmission
and retransmission. Consider two peers, A and B. Suppose A
specifies a Receive Window Size AVP with a value of N in the SCCRQ or
SCCRP message. B is now allowed to have a maximum of N outstanding
(i.e., unacknowledged) control messages. Once N messages have been
sent, B must wait for an acknowledgment from A that advances the
window before sending new control messages. An implementation may
advertise a non-zero receive window as small or as large as it
wishes, depending on its own ability to process incoming messages
before sending an acknowledgement. Each peer MUST limit the number
of unacknowledged messages it will send before receiving an
acknowledgement by this Receive Window Size. The actual internal
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RFC 3931 L2TPv3 March 2005
unacknowledged message send-queue depth may be further limited by
local resource allocation or by dynamic slow-start and congestion-
avoidance mechanisms.
When retransmitting control messages, a slow start and congestion
avoidance window adjustment procedure SHOULD be utilized. A
recommended procedure is described in Appendix A. A peer MAY drop
messages, but MUST NOT actively delay acknowledgment of messages as a
technique for flow control of control messages. Appendix B contains
examples of control message transmission, acknowledgment, and
retransmission.
4.3. Control Message Authentication
L2TP incorporates an optional authentication and integrity check for
all control messages. This mechanism consists of a computed one-way
hash over the header and body of the L2TP control message, a pre-
configured shared secret, and a local and remote nonce (random value)
exchanged via the Control Message Authentication Nonce AVP. This
per-message authentication and integrity check is designed to perform
a mutual authentication between L2TP nodes, perform integrity
checking of all control messages, and guard against control message
spoofing and replay attacks that would otherwise be trivial to mount.
At least one shared secret (password) MUST exist between
communicating L2TP nodes to enable Control Message Authentication.
See Section 5.4.3 for details on calculation of the Message Digest
and construction of the Control Message Authentication Nonce and
Message Digest AVPs.
L2TPv3 Control Message Authentication is similar to L2TPv2 [RFC2661]
Tunnel Authentication in its use of a shared secret and one-way hash
calculation. The principal difference is that, instead of computing
the hash over selected contents of a received control message (e.g.,
the Challenge AVP and Message Type) as in L2TPv2, the entire message
is used in the hash in L2TPv3. In addition, instead of including the
hash digest in just the SCCRP and SCCCN messages, it is now included
in all L2TP messages.
The Control Message Authentication mechanism is optional, and may be
disabled if both peers agree. For example, if IPsec is already being
used for security and integrity checking between the LCCEs, the
function of the L2TP mechanism becomes redundant and may be disabled.
Presence of the Control Message Authentication Nonce AVP in an SCCRQ
or SCCRP message serves as indication to a peer that Control Message
Authentication is enabled. If an SCCRQ or SCCRP contains a Control
Message Authentication Nonce AVP, the receiver of the message MUST
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RFC 3931 L2TPv3 March 2005
respond with a Message Digest AVP in all subsequent messages sent.
Control Message Authentication is always bidirectional; either both
sides participate in authentication, or neither does.
If Control Message Authentication is disabled, the Message Digest AVP
still MAY be sent as an integrity check of the message. The
integrity check is calculated as in Section 5.4.3, with an empty
zero-length shared secret, local nonce, and remote nonce. If an
invalid Message Digest is received, it should be assumed that the
message has been corrupted in transit and the message dropped
accordingly.
Implementations MAY rate-limit control messages, particularly SCCRQ
messages, upon receipt for performance reasons or for protection
against denial of service attacks.
4.4. Keepalive (Hello)
L2TP employs a keepalive mechanism to detect loss of connectivity
between a pair of LCCEs. This is accomplished by injecting Hello
control messages (see Section 6.5) after a period of time has elapsed
since the last data message or control message was received on an
L2TP session or control connection, respectively. As with any other
control message, if the Hello message is not reliably delivered, the
sending LCCE declares that the control connection is down and resets
its state for the control connection. This behavior ensures that a
connectivity failure between the LCCEs is detected independently by
each end of a control connection.
Since the control channel is operated in-band with data traffic over
the PSN, this single mechanism can be used to infer basic data
connectivity between a pair of LCCEs for all sessions associated with
the control connection.
Periodic keepalive for the control connection MUST be implemented by
sending a Hello if a period of time (a recommended default is 60
seconds, but MUST be configurable) has passed without receiving any
message (data or control) from the peer. An LCCE sending Hello
messages across multiple control connections between the same LCCE
endpoints MUST employ a jittered timer mechanism to prevent grouping
of Hello messages.
4.5. Forwarding Session Data Frames
Once session establishment is complete, circuit frames are received
at an LCCE, encapsulated in L2TP (with appropriate attention to
framing, as described in documents for the particular pseudowire
type), and forwarded over the appropriate session. For every
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RFC 3931 L2TPv3 March 2005
outgoing data message, the sender places the identifier specified in
the Local Session ID AVP (received from peer during session
establishment) in the Session ID field of the L2TP data header. In
this manner, session frames are multiplexed and demultiplexed between
a given pair of LCCEs. Multiple control connections may exist
between a given pair of LCCEs, and multiple sessions may be
associated with a given control connection.
The peer LCCE receiving the L2TP data packet identifies the session
with which the packet is associated by the Session ID in the data
packet's header. The LCCE then checks the Cookie field in the data
packet against the Cookie value received in the Assigned Cookie AVP
during session establishment. It is important for implementers to
note that the Cookie field check occurs after looking up the session
context by the Session ID, and as such, consists merely of a value
match of the Cookie field and that stored in the retrieved context.
There is no need to perform a lookup across the Session ID and Cookie
as a single value. Any received data packets that contain invalid
Session IDs or associated Cookie values MUST be dropped. Finally,
the LCCE either forwards the network packet within the tunneled frame
(e.g., as an LNS) or switches the frame to a circuit (e.g., as an
LAC).
4.6. Default L2-Specific Sublayer
This document defines a Default L2-Specific Sublayer format (see
Section 3.2.2) that a pseudowire may use for features such as
sequencing support, L2 interworking, OAM, or other per-data-packet
operations. The Default L2-Specific Sublayer SHOULD be used by a
given PW type to support these features if it is adequate, and its
presence is requested by a peer during session negotiation.
Alternative sublayers MAY be defined (e.g., an encapsulation with a
larger Sequence Number field or timing information) and identified
for use via the L2-Specific Sublayer Type AVP.
Figure 4.6: Default L2-Specific Sublayer Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|x|x|x|x| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The S (Sequence) bit is set to 1 when the Sequence Number contains a
valid number for this sequenced frame. If the S bit is set to zero,
the Sequence Number contents are undefined and MUST be ignored by the
receiver.
Lau, et al. Standards Track [Page 27]

RFC 3931 L2TPv3 March 2005
The Sequence Number field contains a free-running counter of 2^24
sequence numbers. If the number in this field is valid, the S bit
MUST be set to 1. The Sequence Number begins at zero, which is a
valid sequence number. (In this way, implementations inserting
sequence numbers do not have to "skip" zero when incrementing.) The
sequence number in the header of a received message is considered
less than or equal to the last received number if its value lies in
the range of the last received number and the preceding (2^23-1)
values, inclusive.
4.6.1. Sequencing Data Packets
The Sequence Number field may be used to detect lost, duplicate, or
out-of-order packets within a given session.
When L2 frames are carried over an L2TP-over-IP or L2TP-over-UDP/IP
data channel, this part of the link has the characteristic of being
able to reorder, duplicate, or silently drop packets. Reordering may
break some non-IP protocols or L2 control traffic being carried by
the link. Silent dropping or duplication of packets may break
protocols that assume per-packet indications of error, such as TCP
header compression. While a common mechanism for packet sequence
detection is provided, the sequence dependency characteristics of
individual protocols are outside the scope of this document.
If any protocol being transported by over L2TP data channels cannot
tolerate misordering of data packets, packet duplication, or silent
packet loss, sequencing may be enabled on some or all packets by
using the S bit and Sequence Number field defined in the Default L2-
Specific Sublayer (see Section 4.6). For a given L2TP session, each
LCCE is responsible for communicating to its peer the level of
sequencing support that it requires of data packets that it receives.
Mechanisms to advertise this information during session negotiation
are provided (see Data Sequencing AVP in Section 5.4.4).
When determining whether a packet is in or out of sequence, an
implementation SHOULD utilize a method that is resilient to temporary
dropouts in connectivity coupled with high per-session packet rates.
The recommended method is outlined in Appendix C.
4.7. L2TPv2/v3 Interoperability and Migration
L2TPv2 and L2TPv3 environments should be able to coexist while a
migration to L2TPv3 is made. Migration issues are discussed for each
media type in this section. Most issues apply only to
implementations that require both L2TPv2 and L2TPv3 operation.
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RFC 3931 L2TPv3 March 2005
However, even L2TPv3-only implementations must at least be mindful of
these issues in order to interoperate with implementations that
support both versions.
4.7.1. L2TPv3 over IP
L2TPv3 implementations running strictly over IP with no desire to
interoperate with L2TPv2 implementations may safely disregard most
migration issues from L2TPv2. All control messages and data messages
are sent as described in this document, without normative reference
to RFC 2661.
If one wishes to tunnel PPP over L2TPv3, and fallback to L2TPv2 only
if it is not available, then L2TPv3 over UDP with automatic fallback
(see Section 4.7.3) MUST be used. There is no deterministic method
for automatic fallback from L2TPv3 over IP to either L2TPv2 or L2TPv3
over UDP. One could infer whether L2TPv3 over IP is supported by
sending an SCCRQ and waiting for a response, but this could be
problematic during periods of packet loss between L2TP nodes.
4.7.2. L2TPv3 over UDP
The format of the L2TPv3 over UDP header is defined in Section4.1.2.1.
When operating over UDP, L2TPv3 uses the same port (1701) as L2TPv2
and shares the first two octets of header format with L2TPv2. The
Ver field is used to distinguish L2TPv2 packets from L2TPv3 packets.
If an implementation is capable of operating in L2TPv2 or L2TPv3
modes, it is possible to automatically detect whether a peer can
support L2TPv2 or L2TPv3 and operate accordingly. The details of
this fallback capability is defined in the following section.
4.7.3. Automatic L2TPv2 Fallback
When running over UDP, an implementation may detect whether a peer is
L2TPv3-capable by sending a special SCCRQ that is properly formatted
for both L2TPv2 and L2TPv3. This is accomplished by sending an SCCRQ
with its Ver field set to 2 (for L2TPv2), and ensuring that any
L2TPv3-specific AVPs (i.e., AVPs present within this document and not
defined within RFC 2661) in the message are sent with each M bit set
to 0, and that all L2TPv2 AVPs are present as they would be for
L2TPv2. This is done so that L2TPv3 AVPs will be ignored by an
L2TPv2-only implementation. Note that, in both L2TPv2 and L2TPv3,
the value contained in the space of the control message header
utilized by the 32-bit Control Connection ID in L2TPv3, and the 16-
bit Tunnel ID and
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RFC 3931 L2TPv3 March 2005
16-bit Session ID in L2TPv2, are always 0 for an SCCRQ. This
effectively hides the fact that there are a pair of 16-bit fields in
L2TPv2, and a single 32-bit field in L2TPv3.
If the peer implementation is L2TPv3-capable, a control message with
the Ver field set to 3 and an L2TPv3 header and message format will
be sent in response to the SCCRQ. Operation may then continue as
L2TPv3. If a message is received with the Ver field set to 2, it
must be assumed that the peer implementation is L2TPv2-only, thus
enabling fallback to L2TPv2 mode to safely occur.
Note Well: The L2TPv2/v3 auto-detection mode requires that all L2TPv3
implementations over UDP be liberal in accepting an SCCRQ control
message with the Ver field set to 2 or 3 and the presence of L2TPv2-
specific AVPs. An L2TPv3-only implementation MUST ignore all L2TPv2
AVPs (e.g., those defined in RFC 2661 and not in this document)
within an SCCRQ with the Ver field set to 2 (even if the M bit is set
on the L2TPv2-specific AVPs).
5. Control Message Attribute Value Pairs
To maximize extensibility while permitting interoperability, a
uniform method for encoding message types is used throughout L2TP.
This encoding will be termed AVP (Attribute Value Pair) for the
remainder of this document.
5.1. AVP Format
Each AVP is encoded as follows:
Figure 5.1: AVP Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Type | Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(until Length is reached) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first six bits comprise a bit mask that describes the general
attributes of the AVP. Two bits are defined in this document; the
remaining bits are reserved for future extensions. Reserved bits
MUST be set to 0 when sent and ignored upon receipt.
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RFC 3931 L2TPv3 March 2005
Mandatory (M) bit: Controls the behavior required of an
implementation that receives an unrecognized AVP. The M bit of a
given AVP MUST only be inspected and acted upon if the AVP is
unrecognized (see Section 5.2).
Hidden (H) bit: Identifies the hiding of data in the Attribute Value
field of an AVP. This capability can be used to avoid the passing of
sensitive data, such as user passwords, as cleartext in an AVP.
Section 5.3 describes the procedure for performing AVP hiding.
Length: Contains the number of octets (including the Overall Length
and bit mask fields) contained in this AVP. The Length may be
calculated as 6 + the length of the Attribute Value field in octets.
The field itself is 10 bits, permitting a maximum of 1023 octets of
data in a single AVP. The minimum Length of an AVP is 6. If the
Length is 6, then the Attribute Value field is absent.
Vendor ID: The IANA-assigned "SMI Network Management Private
Enterprise Codes" [RFC1700] value. The value 0, corresponding to
IETF-adopted attribute values, is used for all AVPs defined within
this document. Any vendor wishing to implement its own L2TP
extensions can use its own Vendor ID along with private Attribute
values, guaranteeing that they will not collide with any other
vendor's extensions or future IETF extensions. Note that there are
16 bits allocated for the Vendor ID, thus limiting this feature to
the first 65,535 enterprises.
Attribute Type: A 2-octet value with a unique interpretation across
all AVPs defined under a given Vendor ID.
Attribute Value: This is the actual value as indicated by the Vendor
ID and Attribute Type. It follows immediately after the Attribute
Type field and runs for the remaining octets indicated in the Length
(i.e., Length minus 6 octets of header). This field is absent if the
Length is 6.
In the event that the 16-bit Vendor ID space is exhausted, vendor-
specific AVPs with a 32-bit Vendor ID MUST be encapsulated in the
following manner:
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RFC 3931 L2TPv3 March 2005
Figure 5.2: Extended Vendor ID AVP Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 58 | 32-bit Vendor ID ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(until Length is reached) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP encodes a vendor-specific AVP with a 32-bit Vendor ID space
within the Attribute Value field. Multiple AVPs of this type may
exist in any message. The 16-bit Vendor ID MUST be 0, indicating
that this is an IETF-defined AVP, and the Attribute Type MUST be 58,
indicating that what follows is a vendor-specific AVP with a 32-bit
Vendor ID code. This AVP MAY be hidden (the H bit MAY be 0 or 1).
The M bit for this AVP MUST be set to 0. The Length of the AVP is 12
plus the length of the Attribute Value.
5.2. Mandatory AVPs and Setting the M Bit
If the M bit is set on an AVP that is unrecognized by its recipient,
the session or control connection associated with the control message
containing the AVP MUST be shut down. If the control message
containing the unrecognized AVP is associated with a session (e.g.,
an ICRQ, ICRP, ICCN, SLI, etc.), then the session MUST be issued a
CDN with a Result Code of 2 and Error Code of 8 (as defined in
Section 5.4.2) and shut down. If the control message containing the
unrecognized AVP is associated with establishment or maintenance of a
Control Connection (e.g., SCCRQ, SCCRP, SCCCN, Hello), then the
associated Control Connection MUST be issued a StopCCN with Result
Code of 2 and Error Code of 8 (as defined in Section 5.4.2) and shut
down. If the M bit is not set on an unrecognized AVP, the AVP MUST
be ignored when received, processing the control message as if the
AVP were not present.
Receipt of an unrecognized AVP that has the M bit set is catastrophic
to the session or control connection with which it is associated.
Thus, the M bit should only be set for AVPs that are deemed crucial
to proper operation of the session or control connection by the
sender. AVPs that are considered crucial by the sender may vary by
application and configured options. In no case shall a receiver of
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an AVP "validate" if the M bit is set on a recognized AVP. If the
AVP is recognized (as all AVPs defined in this document MUST be for a
compliant L2TPv3 specification), then by definition, the M bit is of
no consequence.
The sender of an AVP is free to set its M bit to 1 or 0 based on
whether the configured application strictly requires the value
contained in the AVP to be recognized or not. For example,
"Automatic L2TPv2 Fallback" in Section 4.7.3 requires the setting of
the M bit on all new L2TPv3 AVPs to zero if fallback to L2TPv2 is
supported and desired, and 1 if not.
The M bit is useful as extra assurance for support of critical AVP
extensions. However, more explicit methods may be available to
determine support for a given feature rather than using the M bit
alone. For example, if a new AVP is defined in a message for which
there is always a message reply (i.e., an ICRQ, ICRP, SCCRQ, or SCCRP
message), rather than simply sending an AVP in the message with the M
bit set, availability of the extension may be identified by sending
an AVP in the request message and expecting a corresponding AVP in a
reply message. This more explicit method, when possible, is
preferred.
The M bit also plays a role in determining whether or not a malformed
or out-of-range value within an AVP should be ignored or should
result in termination of a session or control connection (see Section7.1 for more details).
5.3. Hiding of AVP Attribute Values
The H bit in the header of each AVP provides a mechanism to indicate
to the receiving peer whether the contents of the AVP are hidden or
present in cleartext. This feature can be used to hide sensitive
control message data such as user passwords, IDs, or other vital
information.
The H bit MUST only be set if (1) a shared secret exists between the
LCCEs and (2) Control Message Authentication is enabled (see Section4.3). If the H bit is set in any AVP(s) in a given control message,
at least one Random Vector AVP must also be present in the message
and MUST precede the first AVP having an H bit of 1.
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The shared secret between LCCEs is used to derive a unique shared key
for hiding and unhiding calculations. The derived shared key is
obtained via an HMAC-MD5 keyed hash [RFC2104], with the key
consisting of the shared secret, and with the data being hashed
consisting of a single octet containing the value 1.
shared_key = HMAC_MD5 (shared_secret, 1)
Hiding an AVP value is done in several steps. The first step is to
take the length and value fields of the original (cleartext) AVP and
encode them into the Hidden AVP Subformat, which appears as follows:
Figure 5.3: Hidden AVP Subformat
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Original Value | Original Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Padding ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length of Original Attribute Value: This is length of the Original
Attribute Value to be obscured in octets. This is necessary to
determine the original length of the Attribute Value that is lost
when the additional Padding is added.
Original Attribute Value: Attribute Value that is to be obscured.
Padding: Random additional octets used to obscure length of the
Attribute Value that is being hidden.
To mask the size of the data being hidden, the resulting subformat
MAY be padded as shown above. Padding does NOT alter the value
placed in the Length of Original Attribute Value field, but does
alter the length of the resultant AVP that is being created. For
example, if an Attribute Value to be hidden is 4 octets in length,
the unhidden AVP length would be 10 octets (6 + Attribute Value
length). After hiding, the length of the AVP would become 6 +
Attribute Value length + size of the Length of Original Attribute
Value field + Padding. Thus, if Padding is 12 octets, the AVP length
would be 6 + 4 + 2 + 12 = 24 octets.
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Next, an MD5 [RFC1321] hash is performed (in network byte order) on
the concatenation of the following:
+ the 2-octet Attribute number of the AVP
+ the shared key
+ an arbitrary length random vector
The value of the random vector used in this hash is passed in the
value field of a Random Vector AVP. This Random Vector AVP must be
placed in the message by the sender before any hidden AVPs. The same
random vector may be used for more than one hidden AVP in the same
message, but not for hiding two or more instances of an AVP with the
same Attribute Type unless the Attribute Values in the two AVPs are
also identical. When a different random vector is used for the
hiding of subsequent AVPs, a new Random Vector AVP MUST be placed in
the control message before the first AVP to which it applies.
The MD5 hash value is then XORed with the first 16-octet (or less)
segment of the Hidden AVP Subformat and placed in the Attribute Value
field of the Hidden AVP. If the Hidden AVP Subformat is less than 16
octets, the Subformat is transformed as if the Attribute Value field
had been padded to 16 octets before the XOR. Only the actual octets
present in the Subformat are modified, and the length of the AVP is
not altered.
If the Subformat is longer than 16 octets, a second one-way MD5 hash
is calculated over a stream of octets consisting of the shared key
followed by the result of the first XOR. That hash is XORed with the
second 16-octet (or less) segment of the Subformat and placed in the
corresponding octets of the Value field of the Hidden AVP.
If necessary, this operation is repeated, with the shared key used
along with each XOR result to generate the next hash to XOR the next
segment of the value with.
The hiding method was adapted from [RFC2865], which was taken from
the "Mixing in the Plaintext" section in the book "Network Security"
by Kaufman, Perlman and Speciner [KPS]. A detailed explanation of
the method follows:
Call the shared key S, the Random Vector RV, and the Attribute Type
A. Break the value field into 16-octet chunks p_1, p_2, etc., with
the last one padded at the end with random data to a 16-octet
boundary. Call the ciphertext blocks c_1, c_2, etc. We will also
define intermediate values b_1, b_2, etc.
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b_1 = MD5 (A + S + RV) c_1 = p_1 xor b_1
b_2 = MD5 (S + c_1) c_2 = p_2 xor b_2
. .
. .
. .
b_i = MD5 (S + c_i-1) c_i = p_i xor b_i
The String will contain c_1 + c_2 +...+ c_i, where "+" denotes
concatenation.
On receipt, the random vector is taken from the last Random Vector
AVP encountered in the message prior to the AVP to be unhidden. The
above process is then reversed to yield the original value.
5.4. AVP Summary
The following sections contain a list of all L2TP AVPs defined in
this document.
Following the name of the AVP is a list indicating the message types
that utilize each AVP. After each AVP title follows a short
description of the purpose of the AVP, a detail (including a graphic)
of the format for the Attribute Value, and any additional information
needed for proper use of the AVP.
5.4.1. General Control Message AVPs
Message Type (All Messages)
The Message Type AVP, Attribute Type 0, identifies the control
message herein and defines the context in which the exact meaning
of the following AVPs will be determined.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type is a 2-octet unsigned integer.
The Message Type AVP MUST be the first AVP in a message,
immediately following the control message header (defined in
Section 3.2.1). See Section 3.1 for the list of defined control
message types and their identifiers.
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The Mandatory (M) bit within the Message Type AVP has special
meaning. Rather than an indication as to whether the AVP itself
should be ignored if not recognized, it is an indication as to
whether the control message itself should be ignored. If the M
bit is set within the Message Type AVP and the Message Type is
unknown to the implementation, the control connection MUST be
cleared. If the M bit is not set, then the implementation may
ignore an unknown message type. The M bit MUST be set to 1 for
all message types defined in this document. This AVP MUST NOT be
hidden (the H bit MUST be 0). The Length of this AVP is 8.
A vendor-specific control message may be defined by setting the
Vendor ID of the Message Type AVP to a value other than the IETF
Vendor ID of 0 (see Section 5.1). The Message Type AVP MUST still
be the first AVP in the control message.
Message Digest (All Messages)
The Message Digest AVP, Attribute Type 59 is used as an integrity
and authentication check of the L2TP Control Message header and
body.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest Type | Message Digest ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (16 or 20 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Digest Type is a one-octet integer indicating the Digest
calculation algorithm:
0 HMAC-MD5 [RFC2104]
1 HMAC-SHA-1 [RFC2104]
Digest Type 0 (HMAC-MD5) MUST be supported, while Digest Type 1
(HMAC-SHA-1) SHOULD be supported.
The Message Digest is of variable length and contains the result
of the control message authentication and integrity calculation.
For Digest Type 0 (HMAC-MD5), the length of the digest MUST be 16
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bytes. For Digest Type 1 (HMAC-SHA-1) the length of the digest
MUST be 20 bytes.
If Control Message Authentication is enabled, at least one Message
Digest AVP MUST be present in all messages and MUST be placed
immediately after the Message Type AVP. This forces the Message
Digest AVP to begin at a well-known and fixed offset. A second
Message Digest AVP MAY be present in a message and MUST be placed
directly after the first Message Digest AVP.
The shared secret between LCCEs is used to derive a unique shared
key for Control Message Authentication calculations. The derived
shared key is obtained via an HMAC-MD5 keyed hash [RFC2104], with
the key consisting of the shared secret, and with the data being
hashed consisting of a single octet containing the value 2.
shared_key = HMAC_MD5 (shared_secret, 2)
Calculation of the Message Digest is as follows for all messages
other than the SCCRQ (where "+" refers to concatenation):
Message Digest = HMAC_Hash (shared_key, local_nonce +
remote_nonce + control_message)
HMAC_Hash: HMAC Hashing algorithm identified by the Digest Type
(MD5 or SHA1)
local_nonce: Nonce chosen locally and advertised to the remote
LCCE.
remote_nonce: Nonce received from the remote LCCE
(The local_nonce and remote_nonce are advertised via the
Control Message Authentication Nonce AVP, also defined in this
section.)
shared_key: Derived shared key for this control connection
control_message: The entire contents of the L2TP control
message, including the control message header and all AVPs.
Note that the control message header in this case begins after
the all-zero Session ID when running over IP (see Section4.1.1.2), and after the UDP header when running over UDP (see
Section 4.1.2.1).
When calculating the Message Digest, the Message Digest AVP MUST
be present within the control message with the Digest Type set to
its proper value, but the Message Digest itself set to zeros.
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When receiving a control message, the contents of the Message
Digest AVP MUST be compared against the expected digest value
based on local calculation. This is done by performing the same
digest calculation above, with the local_nonce and remote_nonce
reversed. This message authenticity and integrity checking MUST
be performed before utilizing any information contained within the
control message. If the calculation fails, the message MUST be
dropped.
The SCCRQ has special treatment as it is the initial message
commencing a new control connection. As such, there is only one
nonce available. Since the nonce is present within the message
itself as part of the Control Message Authentication Nonce AVP,
there is no need to use it in the calculation explicitly.
Calculation of the SCCRQ Message Digest is performed as follows:
Message Digest = HMAC_Hash (shared_key, control_message)
To allow for graceful switchover to a new shared secret or hash
algorithm, two Message Digest AVPs MAY be present in a control
message, and two shared secrets MAY be configured for a given
LCCE. If two Message Digest AVPs are received in a control
message, the message MUST be accepted if either Message Digest is
valid. If two shared secrets are configured, each (separately)
MUST be used for calculating a digest to be compared to the
Message Digest(s) received. When calculating a digest for a
control message, the Value field for both of the Message Digest
AVPs MUST be set to zero.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length is 23 for Digest Type 1 (HMAC-MD5), and 27 for Digest Type
2 (HMAC-SHA-1).
Control Message Authentication Nonce (SCCRQ, SCCRP)
The Control Message Authentication Nonce AVP, Attribute Type 73,
MUST contain a cryptographically random value [RFC1750]. This
value is used for Control Message Authentication.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The Nonce is of arbitrary length, though at least 16 octets is
recommended. The Nonce contains the random value for use in the
Control Message Authentication hash calculation (see Message
Digest AVP definition in this section).
If Control Message Authentication is enabled, this AVP MUST be
present in the SCCRQ and SCCRP messages.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Nonce.
Random Vector (All Messages)
The Random Vector AVP, Attribute Type 36, MUST contain a
cryptographically random value [RFC1750]. This value is used for
AVP Hiding.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random Octet String ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Random Octet String is of arbitrary length, though at least 16
octets is recommended. The string contains the random vector for
use in computing the MD5 hash to retrieve or hide the Attribute
Value of a hidden AVP (see Section 5.3).
More than one Random Vector AVP may appear in a message, in which
case a hidden AVP uses the Random Vector AVP most closely
preceding it. As such, at least one Random Vector AVP MUST
precede the first AVP with the H bit set.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Random Octet
String.
5.4.2. Result and Error Codes
Result Code (StopCCN, CDN)
The Result Code AVP, Attribute Type 1, indicates the reason for
terminating the control connection or session.
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The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code | Error Code (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Message ... (optional, arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code is a 2-octet unsigned integer. The optional Error
Code is a 2-octet unsigned integer. An optional Error Message can
follow the Error Code field. Presence of the Error Code and
Message is indicated by the AVP Length field. The Error Message
contains an arbitrary string providing further (human-readable)
text associated with the condition. Human-readable text in all
error messages MUST be provided in the UTF-8 charset [RFC3629]
using the Default Language [RFC2277].
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length is 8 if there is no Error Code or Message, 10 if there is
an Error Code and no Error Message, or 10 plus the length of the
Error Message if there is an Error Code and Message.
Defined Result Code values for the StopCCN message are as follows:
0 - Reserved.
1 - General request to clear control connection.
2 - General error, Error Code indicates the problem.
3 - Control connection already exists.
4 - Requester is not authorized to establish a control
connection.
5 - The protocol version of the requester is not supported,
Error Code indicates highest version supported.
6 - Requester is being shut down.
7 - Finite state machine error or timeout
General Result Code values for the CDN message are as follows:
0 - Reserved.
1 - Session disconnected due to loss of carrier or
circuit disconnect.
2 - Session disconnected for the reason indicated in Error
Code.
3 - Session disconnected for administrative reasons.
4 - Session establishment failed due to lack of appropriate
facilities being available (temporary condition).
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5 - Session establishment failed due to lack of appropriate
facilities being available (permanent condition).
13 - Session not established due to losing tie breaker.
14 - Session not established due to unsupported PW type.
15 - Session not established, sequencing required without
valid L2-Specific Sublayer.
16 - Finite state machine error or timeout.
Additional service-specific Result Codes are defined outside this
document.
The Error Codes defined below pertain to types of errors that are
not specific to any particular L2TP request, but rather to
protocol or message format errors. If an L2TP reply indicates in
its Result Code that a General Error occurred, the General Error
value should be examined to determine what the error was. The
currently defined General Error codes and their meanings are as
follows:
0 - No General Error.
1 - No control connection exists yet for this pair of LCCEs.
2 - Length is wrong.
3 - One of the field values was out of range.
4 - Insufficient resources to handle this operation now.
5 - Invalid Session ID.
6 - A generic vendor-specific error occurred.
7 - Try another. If initiator is aware of other possible
responder destinations, it should try one of them. This can
be used to guide an LAC or LNS based on policy.
8 - The session or control connection was shut down due to receipt
of an unknown AVP with the M bit set (see Section 5.2). The
Error Message SHOULD contain the attribute of the offending
AVP in (human-readable) text form.
9 - Try another directed. If an LAC or LNS is aware of other
possible destinations, it should inform the initiator of the
control connection or session. The Error Message MUST contain
a comma-separated list of addresses from which the initiator
may choose. If the L2TP data channel runs over IPv4, then
this would be a comma-separated list of IP addresses in the
canonical dotted-decimal format (e.g., "192.0.2.1, 192.0.2.2,
192.0.2.3") in the UTF-8 charset [RFC3629] using the Default
Language [RFC2277]. If there are no servers for the LAC or
LNS to suggest, then Error Code 7 should be used. For IPv4,
the delimiter between addresses MUST be precisely a single
comma and a single space. For IPv6, each literal address MUST
be enclosed in "[" and "]" characters, following the encoding
described in [RFC2732].
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When a General Error Code of 6 is used, additional information
about the error SHOULD be included in the Error Message field. A
vendor-specific AVP MAY be sent to more precisely detail a
vendor-specific problem.
5.4.3. Control Connection Management AVPs
Control Connection Tie Breaker (SCCRQ)
The Control Connection Tie Breaker AVP, Attribute Type 5,
indicates that the sender desires a single control connection to
exist between a given pair of LCCEs.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Control Connection Tie Breaker Value is an 8-octet random
value that is used to choose a single control connection when two
LCCEs request a control connection concurrently. The recipient of
a SCCRQ must check to see if a SCCRQ has been sent to the peer; if
so, a tie has been detected. In this case, the LCCE must compare
its Control Connection Tie Breaker value with the one received in
the SCCRQ. The lower value "wins", and the "loser" MUST discard
its control connection. A StopCCN SHOULD be sent by the winner as
an explicit rejection for the losing SCCRQ. In the case in which
a tie breaker is present on both sides and the value is equal,
both sides MUST discard their control connections and restart
control connection negotiation with a new, random tie breaker
value.
If a tie breaker is received and an outstanding SCCRQ has no tie
breaker value, the initiator that included the Control Connection
Tie Breaker AVP "wins". If neither side issues a tie breaker,
then two separate control connections are opened.
Applications that employ a distinct and well-known initiator have
no need for tie breaking, and MAY omit this AVP or disable tie
breaking functionality. Applications that require tie breaking
also require that an LCCE be uniquely identifiable upon receipt of
an SCCRQ. For L2TP over IP, this MUST be accomplished via the
Router ID AVP.
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Note that in [RFC2661], this AVP is referred to as the "Tie
Breaker AVP" and is applicable only to a control connection. In
L2TPv3, the AVP serves the same purpose of tie breaking, but is
applicable to a control connection or a session. The Control
Connection Tie Breaker AVP (present only in Control Connection
messages) and Session Tie Breaker AVP (present only in Session
messages), are described separately in this document, but share
the same Attribute type of 5.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
length of this AVP is 14.
Host Name (SCCRQ, SCCRP)
The Host Name AVP, Attribute Type 7, indicates the name of the
issuing LAC or LNS, encoded in the US-ASCII charset.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Host Name is of arbitrary length, but MUST be at least 1
octet.
This name should be as broadly unique as possible; for hosts
participating in DNS [RFC1034], a host name with fully qualified
domain would be appropriate. The Host Name AVP and/or Router ID
AVP MUST be used to identify an LCCE as described in Section 3.3.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Host Name.
Router ID (SCCRQ, SCCRP)
The Router ID AVP, Attribute Type 60, is an identifier used to
identify an LCCE for control connection setup, tie breaking,
and/or tunnel authentication.
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The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Router Identifier is a 4-octet unsigned integer. Its value is
unique for a given LCCE, per Section 8.1 of [RFC2072]. The Host
Name AVP and/or Router ID AVP MUST be used to identify an LCCE as
described in Section 3.3.
Implementations MUST NOT assume that Router Identifier is a valid
IP address. The Router Identifier for L2TP over IPv6 can be
obtained from an IPv4 address (if available) or via unspecified
implementation-specific means.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 10.
Vendor Name (SCCRQ, SCCRP)
The Vendor Name AVP, Attribute Type 8, contains a vendor-specific
(possibly human-readable) string describing the type of LAC or LNS
being used.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Vendor Name is the indicated number of octets representing the
vendor string. Human-readable text for this AVP MUST be provided
in the US-ASCII charset [RFC1958, RFC2277].
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 6 plus the length of the
Vendor Name.
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RFC 3931 L2TPv3 March 2005
Assigned Control Connection ID (SCCRQ, SCCRP, StopCCN)
The Assigned Control Connection ID AVP, Attribute Type 61,
contains the ID being assigned to this control connection by the
sender.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Control Connection ID is a 4-octet non-zero unsigned
integer.
The Assigned Control Connection ID AVP establishes the identifier
used to multiplex and demultiplex multiple control connections
between a pair of LCCEs. Once the Assigned Control Connection ID
AVP has been received by an LCCE, the Control Connection ID
specified in the AVP MUST be included in the Control Connection ID
field of all control packets sent to the peer for the lifetime of
the control connection. Before the Assigned Control Connection ID
AVP is received from a peer, all control messages MUST be sent to
that peer with a Control Connection ID value of 0 in the header.
Because a Control Connection ID value of 0 is used in this special
manner, the zero value MUST NOT be sent as an Assigned Control
Connection ID value.
Under certain circumstances, an LCCE may need to send a StopCCN to
a peer without having yet received an Assigned Control Connection
ID AVP from the peer (i.e., SCCRQ sent, no SCCRP received yet).
In this case, the Assigned Control Connection ID AVP that had been
sent to the peer earlier (i.e., in the SCCRQ) MUST be sent as the
Assigned Control Connection ID AVP in the StopCCN. This policy
allows the peer to try to identify the appropriate control
connection via a reverse lookup.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 10.
Receive Window Size (SCCRQ, SCCRP)
The Receive Window Size AVP, Attribute Type 10, specifies the
receive window size being offered to the remote peer.
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RFC 3931 L2TPv3 March 2005
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Window Size is a 2-octet unsigned integer.
If absent, the peer must assume a Window Size of 4 for its
transmit window.
The remote peer may send the specified number of control messages
before it must wait for an acknowledgment. See Section 4.2 for
more information on reliable control message delivery.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 8.
Pseudowire Capabilities List (SCCRQ, SCCRP)
The Pseudowire Capabilities List (PW Capabilities List) AVP,
Attribute Type 62, indicates the L2 payload types the sender can
support. The specific payload type of a given session is
identified by the Pseudowire Type AVP.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type 0 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | PW Type N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Defined PW types that may appear in this list are managed by IANA
and will appear in associated pseudowire-specific documents for
each PW type.
If a sender includes a given PW type in the PW Capabilities List
AVP, the sender assumes full responsibility for supporting that
particular payload, such as any payload-specific AVPs, L2-Specific
Sublayer, or control messages that may be defined in the
appropriate companion document.
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RFC 3931 L2TPv3 March 2005
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 8 octets with one PW type
specified, plus 2 octets for each additional PW type.
Preferred Language (SCCRQ, SCCRP)
The Preferred Language AVP, Attribute Type 72, provides a method
for an LCCE to indicate to the peer the language in which human-
readable messages it sends SHOULD be composed. This AVP contains
a single language tag or language range [RFC3066].
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Language... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Preferred Language is the indicated number of octets
representing the language tag or language range, encoded in the
US-ASCII charset.
It is not required to send a Preferred Language AVP. If (1) an
LCCE does not signify a language preference by the inclusion of
this AVP in the SCCRQ or SCCRP, (2) the Preferred Language AVP is
unrecognized, or (3) the requested language is not supported by
the peer LCCE, the default language [RFC2277] MUST be used for all
internationalized strings sent by the peer.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 6 plus the length of the
Preferred Language.
5.4.4. Session Management AVPs
Local Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Local Session ID AVP (analogous to the Assigned Session ID in
L2TPv2), Attribute Type 63, contains the identifier being assigned
to this session by the sender.
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RFC 3931 L2TPv3 March 2005
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Local Session ID is a 4-octet non-zero unsigned integer.
The Local Session ID AVP establishes the two identifiers used to
multiplex and demultiplex sessions between two LCCEs. Each LCCE
chooses any free value it desires, and sends it to the remote LCCE
using this AVP. The remote LCCE MUST then send all data packets
associated with this session using this value. Additionally, for
all session-oriented control messages sent after this AVP is
received (e.g., ICRP, ICCN, CDN, SLI, etc.), the remote LCCE MUST
echo this value in the Remote Session ID AVP.
Note that a Session ID value is unidirectional. Because each LCCE
chooses its Session ID independent of its peer LCCE, the value
does not have to match in each direction for a given session.
See Section 4.1 for additional information about the Session ID.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be 1 set to 1, but MAY vary (see Section 5.2).
The Length (before hiding) of this AVP is 10.
Remote Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Remote Session ID AVP, Attribute Type 64, contains the
identifier that was assigned to this session by the peer.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote Session ID is a 4-octet non-zero unsigned integer.
The Remote Session ID AVP MUST be present in all session-level
control messages. The AVP's value echoes the session identifier
advertised by the peer via the Local Session ID AVP. It is the
same value that will be used in all transmitted data messages by
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RFC 3931 L2TPv3 March 2005
this side of the session. In most cases, this identifier is
sufficient for the peer to look up session-level context for this
control message.
When a session-level control message must be sent to the peer
before the Local Session ID AVP has been received, the value of
the Remote Session ID AVP MUST be set to zero. Additionally, the
Local Session ID AVP (sent in a previous control message for this
session) MUST be included in the control message. The peer must
then use the Local Session ID AVP to perform a reverse lookup to
find its session context. Session-level control messages defined
in this document that might be subject to a reverse lookup by a
receiving peer include the CDN, WEN, and SLI.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 10.
Assigned Cookie (ICRQ, ICRP, OCRQ, OCRP)
The Assigned Cookie AVP, Attribute Type 65, contains the Cookie
value being assigned to this session by the sender.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Cookie (32 or 64 bits) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Cookie is a 4-octet or 8-octet random value.
The Assigned Cookie AVP contains the value used to check the
association of a received data message with the session identified
by the Session ID. All data messages sent to a peer MUST use the
Assigned Cookie sent by the peer in this AVP. The value's length
(0, 32, or 64 bits) is obtained by the length of the AVP.
A missing Assigned Cookie AVP or Assigned Cookie Value of zero
length indicates that the Cookie field should not be present in
any data packets sent to the LCCE sending this AVP.
See Section 4.1 for additional information about the Assigned
Cookie.
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RFC 3931 L2TPv3 March 2005
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP may be 6, 10, or 14 octets.
Serial Number (ICRQ, OCRQ)
The Serial Number AVP, Attribute Type 15, contains an identifier
assigned by the LAC or LNS to this session.
The Attribute Value field for this AVP has the following format:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Serial Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Serial Number is a 32-bit value.
The Serial Number is intended to be an easy reference for
administrators on both ends of a control connection to use when
investigating session failure problems. Serial Numbers should be
set to progressively increasing values, which are likely to be
unique for a significant period of time across all interconnected
LNSs and LACs.
Note that in RFC 2661, this value was referred to as the "Call
Serial Number AVP". It serves the same purpose and has the same
attribute value and composition.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 10.
Remote End ID (ICRQ, OCRQ)
The Remote End ID AVP, Attribute Type 66, contains an identifier
used to bind L2TP sessions to a given circuit, interface, or
bridging instance. It also may be used to detect session-level
ties.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote End Identifier ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 3931 L2TPv3 March 2005
The Remote End Identifier field is a variable-length field whose
value is unique for a given LCCE peer, as described in Section3.3.
A session-level tie is detected if an LCCE receives an ICRQ or
OCRQ with an End ID AVP whose value matches that which was just
sent in an outgoing ICRQ or OCRQ to the same peer. If the two
values match, an LCCE recognizes that a tie exists (i.e., both
LCCEs are attempting to establish sessions for the same circuit).
The tie is broken by the Session Tie Breaker AVP.
By default, the LAC-LAC cross-connect application (see Section2(b)) of L2TP over an IP network MUST utilize the Router ID AVP
and Remote End ID AVP to associate a circuit to an L2TP session.
Other AVPs MAY be used for LCCE or circuit identification as
specified in companion documents.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 6 plus the length of the
Remote End Identifier value.
Session Tie Breaker (ICRQ, OCRQ)
The Session Tie Breaker AVP, Attribute Type 5, is used to break
ties when two peers concurrently attempt to establish a session
for the same circuit.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Session Tie Breaker Value is an 8-octet random value that is
used to choose a session when two LCCEs concurrently request a
session for the same circuit. A tie is detected by examining the
peer's identity (described in Section 3.3) plus the per-session
shared value communicated via the End ID AVP. In the case of a
tie, the recipient of an ICRQ or OCRQ must compare the received
tie breaker value with the one that it sent earlier. The LCCE
with the lower value "wins" and MUST send a CDN with result code
set to 13 (as defined in Section 5.4.2) in response to the losing
ICRQ or OCRQ. In the case in which a tie is detected, tie
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RFC 3931 L2TPv3 March 2005
breakers are sent by both sides, and the tie breaker values are
equal, both sides MUST discard their sessions and restart session
negotiation with new random tie breaker values.
If a tie is detected but only one side sends a Session Tie Breaker
AVP, the session initiator that included the Session Tie Breaker
AVP "wins". If neither side issues a tie breaker, then both sides
MUST tear down the session.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 14.
Pseudowire Type (ICRQ, OCRQ)
The Pseudowire Type (PW Type) AVP, Attribute Type 68, indicates
the L2 payload type of the packets that will be tunneled using
this L2TP session.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A peer MUST NOT request an incoming or outgoing call with a PW
Type AVP specifying a value not advertised in the PW Capabilities
List AVP it received during control connection establishment.
Attempts to do so MUST result in the call being rejected via a CDN
with the Result Code set to 14 (see Section 5.4.2).
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 8.
L2-Specific Sublayer (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The L2-Specific Sublayer AVP, Attribute Type 69, indicates the
presence and format of the L2-Specific Sublayer the sender of this
AVP requires on all incoming data packets for this L2TP session.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Specific Sublayer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 3931 L2TPv3 March 2005
The L2-Specific Sublayer Type is a 2-octet unsigned integer with
the following values defined in this document:
0 - There is no L2-Specific Sublayer present.
1 - The Default L2-Specific Sublayer (defined in Section 4.6)
is used.
If this AVP is received and has a value other than zero, the
receiving LCCE MUST include the identified L2-Specific Sublayer in
its outgoing data messages. If the AVP is not received, it is
assumed that there is no sublayer present.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 8.
Data Sequencing (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Data Sequencing AVP, Attribute Type 70, indicates that the
sender requires some or all of the data packets that it receives
to be sequenced.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sequencing Level |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Data Sequencing Level is a 2-octet unsigned integer indicating
the degree of incoming data traffic that the sender of this AVP
wishes to be marked with sequence numbers.
Defined Data Sequencing Levels are as follows:
0 - No incoming data packets require sequencing.
1 - Only non-IP data packets require sequencing.
2 - All incoming data packets require sequencing.
If a Data Sequencing Level of 0 is specified, there is no need to
send packets with sequence numbers. If sequence numbers are sent,
they will be ignored upon receipt. If no Data Sequencing AVP is
received, a Data Sequencing Level of 0 is assumed.
If a Data Sequencing Level of 1 is specified, only non-IP traffic
carried within the tunneled L2 frame should have sequence numbers
applied. Non-IP traffic here refers to any packets that cannot be
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RFC 3931 L2TPv3 March 2005
classified as an IP packet within their respective L2 framing
(e.g., a PPP control packet or NETBIOS frame encapsulated by Frame
Relay before being tunneled). All traffic that can be classified
as IP MUST be sent with no sequencing (i.e., the S bit in the L2-
Specific Sublayer is set to zero). If a packet is unable to be
classified at all (e.g., because it has been compressed or
encrypted at layer 2) or if an implementation is unable to perform
such classification within L2 frames, all packets MUST be provided
with sequence numbers (essentially falling back to a Data
Sequencing Level of 2).
If a Data Sequencing Level of 2 is specified, all traffic MUST be
sequenced.
Data sequencing may only be requested when there is an L2-Specific
Sublayer present that can provide sequence numbers. If sequencing
is requested without requesting a L2-Specific Sublayer AVP, the
session MUST be disconnected with a Result Code of 15 (see Section5.4.2).
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 8.
Tx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Tx Connect Speed BPS AVP, Attribute Type 74, contains the
speed of the facility chosen for the connection attempt.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed in bps...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...Connect Speed in bps (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Tx Connect Speed BPS is an 8-octet value indicating the speed
in bits per second. A value of zero indicates that the speed is
indeterminable or that there is no physical point-to-point link.
When the optional Rx Connect Speed AVP is present, the value in
this AVP represents the transmit connect speed from the
perspective of the LAC (i.e., data flowing from the LAC to the
remote system). When the optional Rx Connect Speed AVP is NOT
present, the connection speed between the remote system and LAC is
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RFC 3931 L2TPv3 March 2005
assumed to be symmetric and is represented by the single value in
this AVP.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 14.
Rx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Rx Connect Speed AVP, Attribute Type 75, represents the speed
of the connection from the perspective of the LAC (i.e., data
flowing from the remote system to the LAC).
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed in bps...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...Connect Speed in bps (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Connect Speed BPS is an 8-octet value indicating the speed in bits
per second. A value of zero indicates that the speed is
indeterminable or that there is no physical point-to-point link.
Presence of this AVP implies that the connection speed may be
asymmetric with respect to the transmit connect speed given in the
Tx Connect Speed AVP.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 14.
Physical Channel ID (ICRQ, ICRP, OCRP)
The Physical Channel ID AVP, Attribute Type 25, contains the
vendor-specific physical channel number used for a call.
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Physical Channel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 3931 L2TPv3 March 2005
Physical Channel ID is a 4-octet value intended to be used for
logging purposes only.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 10.
5.4.5. Circuit Status AVPs
Circuit Status (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI)
The Circuit Status AVP, Attribute Type 71, indicates the initial
status of or a status change in the circuit to which the session
is bound.
The Attribute Value field for this AVP has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The A (Active) bit indicates whether the circuit is
up/active/ready (1) or down/inactive/not-ready (0).
The N (New) bit indicates whether the circuit status indication is
for a new circuit (1) or an existing circuit (0). Links that have
a similar mechanism available (e.g., Frame Relay) MUST map the
setting of this bit to the associated signaling for that link.
Otherwise, the New bit SHOULD still be set the first time the L2TP
session is established after provisioning.
The remaining bits are reserved for future use. Reserved bits
MUST be set to 0 when sending and ignored upon receipt.
The Circuit Status AVP is used to advertise whether a circuit or
interface bound to an L2TP session is up and ready to send and/or
receive traffic. Different circuit types have different names for
status types. For example, HDLC primary and secondary stations
refer to a circuit as being "Receive Ready" or "Receive Not
Ready", while Frame Relay refers to a circuit as "Active" or
"Inactive". This AVP adopts the latter terminology, though the
concept remains the same regardless of the PW type for the L2TP
session.
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RFC 3931 L2TPv3 March 2005
In the simplest case, the circuit to which this AVP refers is a
single physical interface, port, or circuit, depending on the
application and the session setup. The status indication in this
AVP may then be used to provide simple ILMI interworking for a
variety of circuit types. For virtual or multipoint interfaces,
the Circuit Status AVP is still utilized, but in this case, it
refers to the state of an internal structure or a logical set of
circuits. Each PW-specific companion document MUST specify
precisely how this AVP is translated for each circuit type.
If this AVP is received with a Not Active notification for a given
L2TP session, all data traffic for that session MUST cease (or not
begin) in the direction of the sender of the Circuit Status AVP
until the circuit is advertised as Active.
The Circuit Status MUST be advertised by this AVP in ICRQ, ICRP,
OCRQ, and OCRP messages. Often, the circuit type will be marked
Active when initiated, but subsequently MAY be advertised as
Inactive. This indicates that an L2TP session is to be created,
but that the interface or circuit is still not ready to pass
traffic. The ICCN, OCCN, and SLI control messages all MAY contain
this AVP to update the status of the circuit after establishment
of the L2TP session is requested.
If additional circuit status information is needed for a given PW
type, any new PW-specific AVPs MUST be defined in a separate
document. This AVP is only for general circuit status information
generally applicable to all circuit/interface types.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 8.
Circuit Errors (WEN)
The Circuit Errors AVP, Attribute Type 34, conveys circuit error
information to the peer.
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RFC 3931 L2TPv3 March 2005
The Attribute Value field for this AVP has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hardware Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Buffer Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timeout Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alignment Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following fields are defined:
Reserved: 2 octets of Reserved data is present (providing longword
alignment within the AVP of the following values). Reserved
data MUST be zero on sending and ignored upon receipt.
Hardware Overruns: Number of receive buffer overruns since call
was established.
Buffer Overruns: Number of buffer overruns detected since call was
established.
Timeout Errors: Number of timeouts since call was established.
Alignment Errors: Number of alignment errors since call was
established.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
Length (before hiding) of this AVP is 32.
6. Control Connection Protocol Specification
The following control messages are used to establish, maintain, and
tear down L2TP control connections. All data packets are sent in
network order (high-order octets first). Any "reserved" or "empty"
fields MUST be sent as 0 values to allow for protocol extensibility.
The exchanges in which these messages are involved are outlined in
Section 3.3.
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RFC 3931 L2TPv3 March 20056.1. Start-Control-Connection-Request (SCCRQ)
Start-Control-Connection-Request (SCCRQ) is a control message used to
initiate a control connection between two LCCEs. It is sent by
either the LAC or the LNS to begin the control connection
establishment process.
The following AVPs MUST be present in the SCCRQ:
Message Type
Host Name
Router ID
Assigned Control Connection ID
Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRQ:
Random Vector
Control Message Authentication Nonce
Message Digest
Control Connection Tie Breaker
Vendor Name
Receive Window Size
Preferred Language
6.2. Start-Control-Connection-Reply (SCCRP)
Start-Control-Connection-Reply (SCCRP) is the control message sent in
reply to a received SCCRQ message. The SCCRP is used to indicate
that the SCCRQ was accepted and that establishment of the control
connection should continue.
The following AVPs MUST be present in the SCCRP:
Message Type
Host Name
Router ID
Assigned Control Connection ID
Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRP:
Random Vector
Control Message Authentication Nonce
Message Digest
Vendor Name
Receive Window Size
Preferred Language
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RFC 3931 L2TPv3 March 20056.3. Start-Control-Connection-Connected (SCCCN)
Start-Control-Connection-Connected (SCCCN) is the control message
sent in reply to an SCCRP. The SCCCN completes the control
connection establishment process.
The following AVP MUST be present in the SCCCN:
Message Type
The following AVP MAY be present in the SCCCN:
Random Vector
Message Digest
6.4. Stop-Control-Connection-Notification (StopCCN)
Stop-Control-Connection-Notification (StopCCN) is the control message
sent by either LCCE to inform its peer that the control connection is
being shut down and that the control connection should be closed. In
addition, all active sessions are implicitly cleared (without sending
any explicit session control messages). The reason for issuing this
request is indicated in the Result Code AVP. There is no explicit
reply to the message, only the implicit ACK that is received by the
reliable control message delivery layer.
The following AVPs MUST be present in the StopCCN:
Message Type
Result Code
The following AVPs MAY be present in the StopCCN:
Random Vector
Message Digest
Assigned Control Connection ID
Note that the Assigned Control Connection ID MUST be present if the
StopCCN is sent after an SCCRQ or SCCRP message has been sent.
6.5. Hello (HELLO)
The Hello (HELLO) message is an L2TP control message sent by either
peer of a control connection. This control message is used as a
"keepalive" for the control connection. See Section 4.2 for a
description of the keepalive mechanism.
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RFC 3931 L2TPv3 March 2005
HELLO messages are global to the control connection. The Session ID
in a HELLO message MUST be 0.
The following AVP MUST be present in the HELLO:
Message Type
The following AVP MAY be present in the HELLO:
Random Vector
Message Digest
6.6. Incoming-Call-Request (ICRQ)
Incoming-Call-Request (ICRQ) is the control message sent by an LCCE
to a peer when an incoming call is detected (although the ICRQ may
also be sent as a result of a local event). It is the first in a
three-message exchange used for establishing a session via an L2TP
control connection.
The ICRQ is used to indicate that a session is to be established
between an LCCE and a peer. The sender of an ICRQ provides the peer
with parameter information for the session. However, the sender
makes no demands about how the session is terminated at the peer
(i.e., whether the L2 traffic is processed locally, forwarded, etc.).
The following AVPs MUST be present in the ICRQ:
Message Type
Local Session ID
Remote Session ID
Serial Number
Pseudowire Type
Remote End ID
Circuit Status
The following AVPs MAY be present in the ICRQ:
Random Vector
Message Digest
Assigned Cookie
Session Tie Breaker
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
Rx Connect Speed
Physical Channel ID
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RFC 3931 L2TPv3 March 20056.7. Incoming-Call-Reply (ICRP)
Incoming-Call-Reply (ICRP) is the control message sent by an LCCE in
response to a received ICRQ. It is the second in the three-message
exchange used for establishing sessions within an L2TP control
connection.
The ICRP is used to indicate that the ICRQ was successful and that
the peer should establish (i.e., answer) the incoming call if it has
not already done so. It also allows the sender to indicate specific
parameters about the L2TP session.
The following AVPs MUST be present in the ICRP:
Message Type
Local Session ID
Remote Session ID
Circuit Status
The following AVPs MAY be present in the ICRP:
Random Vector
Message Digest
Assigned Cookie
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
Rx Connect Speed
Physical Channel ID
6.8. Incoming-Call-Connected (ICCN)
Incoming-Call-Connected (ICCN) is the control message sent by the
LCCE that originally sent an ICRQ upon receiving an ICRP from its
peer. It is the final message in the three-message exchange used for
establishing L2TP sessions.
The ICCN is used to indicate that the ICRP was accepted, that the
call has been established, and that the L2TP session should move to
the established state. It also allows the sender to indicate
specific parameters about the established call (parameters that may
not have been available at the time the ICRQ was issued).
The following AVPs MUST be present in the ICCN:
Message Type
Local Session ID
Remote Session ID
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RFC 3931 L2TPv3 March 2005
The following AVPs MAY be present in the ICCN:
Random Vector
Message Digest
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
Rx Connect Speed
Circuit Status
6.9. Outgoing-Call-Request (OCRQ)
Outgoing-Call-Request (OCRQ) is the control message sent by an LCCE
to an LAC to indicate that an outbound call at the LAC is to be
established based on specific destination information sent in this
message. It is the first in a three-message exchange used for
establishing a session and placing a call on behalf of the initiating
LCCE.
Note that a call may be any L2 connection requiring well-known
destination information to be sent from an LCCE to an LAC. This call
could be a dialup connection to the PSTN, an SVC connection, the IP
address of another LCCE, or any other destination dictated by the
sender of this message.
The following AVPs MUST be present in the OCRQ:
Message Type
Local Session ID
Remote Session ID
Serial Number
Pseudowire Type
Remote End ID
Circuit Status
The following AVPs MAY be present in the OCRQ:
Random Vector
Message Digest
Assigned Cookie
Tx Connect Speed
Rx Connect Speed
Session Tie Breaker
L2-Specific Sublayer
Data Sequencing
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RFC 3931 L2TPv3 March 20056.10. Outgoing-Call-Reply (OCRP)
Outgoing-Call-Reply (OCRP) is the control message sent by an LAC to
an LCCE in response to a received OCRQ. It is the second in a
three-message exchange used for establishing a session within an L2TP
control connection.
OCRP is used to indicate that the LAC has been able to attempt the
outbound call. The message returns any relevant parameters regarding
the call attempt. Data MUST NOT be forwarded until the OCCN is
received, which indicates that the call has been placed.
The following AVPs MUST be present in the OCRP:
Message Type
Local Session ID
Remote Session ID
Circuit Status
The following AVPs MAY be present in the OCRP:
Random Vector
Message Digest
Assigned Cookie
L2-Specific Sublayer
Tx Connect Speed
Rx Connect Speed
Data Sequencing
Physical Channel ID
6.11. Outgoing-Call-Connected (OCCN)
Outgoing-Call-Connected (OCCN) is the control message sent by an LAC
to another LCCE after the OCRP and after the outgoing call has been
completed. It is the final message in a three-message exchange used
for establishing a session.
OCCN is used to indicate that the result of a requested outgoing call
was successful. It also provides information to the LCCE who
requested the call about the particular parameters obtained after the
call was established.
The following AVPs MUST be present in the OCCN:
Message Type
Local Session ID
Remote Session ID
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RFC 3931 L2TPv3 March 2005
The following AVPs MAY be present in the OCCN:
Random Vector
Message Digest
L2-Specific Sublayer
Tx Connect Speed
Rx Connect Speed
Data Sequencing
Circuit Status
6.12. Call-Disconnect-Notify (CDN)
The Call-Disconnect-Notify (CDN) is a control message sent by an LCCE
to request disconnection of a specific session. Its purpose is to
inform the peer of the disconnection and the reason for the
disconnection. The peer MUST clean up any resources, and does not
send back any indication of success or failure for such cleanup.
The following AVPs MUST be present in the CDN:
Message Type
Result Code
Local Session ID
Remote Session ID
The following AVP MAY be present in the CDN:
Random Vector
Message Digest
6.13. WAN-Error-Notify (WEN)
The WAN-Error-Notify (WEN) is a control message sent from an LAC to
an LNS to indicate WAN error conditions. The counters in this
message are cumulative. This message should only be sent when an
error occurs, and not more than once every 60 seconds. The counters
are reset when a new call is established.
The following AVPs MUST be present in the WEN:
Message Type
Local Session ID
Remote Session ID
Circuit Errors
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RFC 3931 L2TPv3 March 2005
The following AVP MAY be present in the WEN:
Random Vector
Message Digest
6.14. Set-Link-Info (SLI)
The Set-Link-Info control message is sent by an LCCE to convey link
or circuit status change information regarding the circuit associated
with this L2TP session. For example, if PPP renegotiates LCP at an
LNS or between an LAC and a Remote System, or if a forwarded Frame
Relay VC transitions to Active or Inactive at an LAC, an SLI message
SHOULD be sent to indicate this event. Precise details of when the
SLI is sent, what PW type-specific AVPs must be present, and how
those AVPs should be interpreted by the receiving peer are outside
the scope of this document. These details should be described in the
associated pseudowire-specific documents that require use of this
message.
The following AVPs MUST be present in the SLI:
Message Type
Local Session ID
Remote Session ID
The following AVPs MAY be present in the SLI:
Random Vector
Message Digest
Circuit Status
6.15. Explicit-Acknowledgement (ACK)
The Explicit Acknowledgement (ACK) message is used only to
acknowledge receipt of a message or messages on the control
connection (e.g., for purposes of updating Ns and Nr values).
Receipt of this message does not trigger an event for the L2TP
protocol state machine.
A message received without any AVPs (including the Message Type AVP),
is referred to as a Zero Length Body (ZLB) message, and serves the
same function as the Explicit Acknowledgement. ZLB messages are only
permitted when Control Message Authentication defined in Section 4.3
is not enabled.
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RFC 3931 L2TPv3 March 2005
The following AVPs MAY be present in the ACK message:
Message Type
Message Digest
7. Control Connection State Machines
The state tables defined in this section govern the exchange of
control messages defined in Section 6. Tables are defined for
incoming call placement and outgoing call placement, as well as for
initiation of the control connection itself. The state tables do not
encode timeout and retransmission behavior, as this is handled in the
underlying reliable control message delivery mechanism (see Section4.2).
7.1. Malformed AVPs and Control Messages
Receipt of an invalid or unrecoverable malformed control message
SHOULD be logged appropriately and the control connection cleared to
ensure recovery to a known state. The control connection may then be
restarted by the initiator.
An invalid control message is defined as (1) a message that contains
a Message Type marked as mandatory (see Section 5.4.1) but that is
unknown to the implementation, or (2) a control message that is
received in the wrong state.
Examples of malformed control messages include (1) a message that has
an invalid value in its header, (2) a message that contains an AVP
that is formatted incorrectly or whose value is out of range, and (3)
a message that is missing a required AVP. A control message with a
malformed header MUST be discarded.
When possible, a malformed AVP should be treated as an unrecognized
AVP (see Section 5.2). Thus, an attempt to inspect the M bit SHOULD
be made to determine the importance of the malformed AVP, and thus,
the severity of the malformation to the entire control message. If
the M bit can be reasonably inspected within the malformed AVP and is
determined to be set, then as with an unrecognized AVP, the
associated session or control connection MUST be shut down. If the M
bit is inspected and is found to be 0, the AVP MUST be ignored
(assuming recovery from the AVP malformation is indeed possible).
This policy must not be considered as a license to send malformed
AVPs, but rather, as a guide towards how to handle an improperly
formatted message if one is received. It is impossible to list all
potential malformations of a given message and give advice for each.
One example of a malformed AVP situation that should be recoverable
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RFC 3931 L2TPv3 March 2005
is if the Rx Connect Speed AVP is received with a length of 10 rather
than 14, implying that the connect speed bits-per-second is being
formatted in 4 octets rather than 8. If the AVP does not have its M
bit set (as would typically be the case), this condition is not
considered catastrophic. As such, the control message should be
accepted as though the AVP were not present (though a local error
message may be logged).
In several cases in the following tables, a protocol message is sent,
and then a "clean up" occurs. Note that, regardless of the initiator
of the control connection destruction, the reliable delivery
mechanism must be allowed to run (see Section 4.2) before destroying
the control connection. This permits the control connection
management messages to be reliably delivered to the peer.
Appendix B.1 contains an example of lock-step control connection
establishment.
7.2. Control Connection States
The L2TP control connection protocol is not distinguishable between
the two LCCEs but is distinguishable between the originator and
receiver. The originating peer is the one that first initiates
establishment of the control connection. (In a tie breaker
situation, this is the winner of the tie.) Since either the LAC or
the LNS can be the originator, a collision can occur. See the
Control Connection Tie Breaker AVP in Section 5.4.3 for a description
of this and its resolution.
State Event Action New State
----- ----- ------ ---------
idle Local open Send SCCRQ wait-ctl-reply
request
idle Receive SCCRQ, Send SCCRP wait-ctl-conn
acceptable
idle Receive SCCRQ, Send StopCCN, idle
not acceptable clean up
idle Receive SCCRP Send StopCCN, idle
clean up
idle Receive SCCCN Send StopCCN, idle
clean up
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RFC 3931 L2TPv3 March 2005
The states associated with an LCCE for control connection
establishment are as follows:
idle
Both initiator and recipient start from this state. An initiator
transmits an SCCRQ, while a recipient remains in the idle state
until receiving an SCCRQ.
wait-ctl-reply
The originator checks to see if another connection has been
requested from the same peer, and if so, handles the collision
situation described in Section 5.4.3.
wait-ctl-conn
Awaiting an SCCCN. If the SCCCN is valid, the control connection
is established; otherwise, it is torn down (sending a StopCCN with
the proper result and/or error code).
established
An established connection may be terminated by either a local
condition or the receipt of a StopCCN. In the event of a local
termination, the originator MUST send a StopCCN and clean up the
control connection. If the originator receives a StopCCN, it MUST
also clean up the control connection.
7.3. Incoming Calls
An ICRQ is generated by an LCCE, typically in response to an incoming
call or a local event. Once the LCCE sends the ICRQ, it waits for a
response from the peer. However, it may choose to postpone
establishment of the call (e.g., answering the call, bringing up the
circuit) until the peer has indicated with an ICRP that it will
accept the call. The peer may choose not to accept the call if, for
instance, there are insufficient resources to handle an additional
session.
If the peer chooses to accept the call, it responds with an ICRP.
When the local LCCE receives the ICRP, it attempts to establish the
call. A final call connected message, the ICCN, is sent from the
local LCCE to the peer to indicate that the call states for both
LCCEs should enter the established state. If the call is terminated
before the peer can accept it, a CDN is sent by the local LCCE to
indicate this condition.
When a call transitions to a "disconnected" or "down" state, the call
is cleared normally, and the local LCCE sends a CDN. Similarly, if
the peer wishes to clear a call, it sends a CDN and cleans up its
session.
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RFC 3931 L2TPv3 March 2005
The states associated with the ICRQ sender are as follows:
idle
The LCCE detects an incoming call on one of its interfaces (e.g.,
an analog PSTN line rings, or an ATM PVC is provisioned), or a
local event occurs. The LCCE initiates its control connection
establishment state machine and moves to a state waiting for
confirmation of the existence of a control connection.
wait-control-conn
In this state, the session is waiting for either the control
connection to be opened or for verification that the control
connection is already open. Once an indication that the control
connection has been opened is received, session control messages
may be exchanged. The first of these messages is the ICRQ.
wait-reply
The ICRQ sender receives either (1) a CDN indicating the peer is
not willing to accept the call (general error or do not accept)
and moves back into the idle state, or (2) an ICRP indicating the
call is accepted. In the latter case, the LCCE sends an ICCN and
enters the established state.
established
Data is exchanged over the session. The call may be cleared by
any of the following:
+ An event on the connected interface: The LCCE sends a CDN.
+ Receipt of a CDN: The LCCE cleans up, disconnecting the call.
+ A local reason: The LCCE sends a CDN.
7.3.2. ICRQ Recipient States
State Event Action New State
----- ----- ------ ---------
idle Receive ICRQ, Send ICRP wait-connect
acceptable
idle Receive ICRQ, Send CDN, idle
not acceptable clean up
idle Receive ICRP Send CDN idle
clean up
idle Receive ICCN Clean up idle
wait-connect Receive ICCN, Prepare for established
acceptable data
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wait-connect Receive ICCN, Send CDN, idle
not acceptable clean up
wait-connect Receive ICRQ, Send CDN, idle
ICRP clean up
idle, Receive CDN Clean up idle
wait-connect,
established
wait-connect Local close Send CDN, idle
established request clean up
established Receive ICRQ, Send CDN, idle
ICRP, ICCN clean up
The states associated with the ICRQ recipient are as follows:
idle
An ICRQ is received. If the request is not acceptable, a CDN is
sent back to the peer LCCE, and the local LCCE remains in the idle
state. If the ICRQ is acceptable, an ICRP is sent. The session
moves to the wait-connect state.
wait-connect
The local LCCE is waiting for an ICCN from the peer. Upon receipt
of the ICCN, the local LCCE moves to established state.
established
The session is terminated either by sending a CDN or by receiving
a CDN from the peer. Clean up follows on both sides regardless of
the initiator.
7.4. Outgoing Calls
Outgoing calls instruct an LAC to place a call. There are three
messages for outgoing calls: OCRQ, OCRP, and OCCN. An LCCE first
sends an OCRQ to an LAC to request an outgoing call. The LAC MUST
respond to the OCRQ with an OCRP once it determines that the proper
facilities exist to place the call and that the call is
administratively authorized. Once the outbound call is connected,
the LAC sends an OCCN to the peer indicating the final result of the
call attempt.
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RFC 3931 L2TPv3 March 2005
wait-control- Local close Clean up idle
conn request
The states associated with the OCRQ sender are as follows:
idle, wait-control-conn
When an outgoing call request is initiated, a control connection
is created as described above, if not already present. Once the
control connection is established, an OCRQ is sent to the LAC, and
the session moves into the wait-reply state.
wait-reply
If a CDN is received, the session is cleaned up and returns to
idle state. If an OCRP is received, the call is in progress, and
the session moves to the wait-connect state.
wait-connect
If a CDN is received, the session is cleaned up and returns to
idle state. If an OCCN is received, the call has succeeded, and
the session may now exchange data.
established
If a CDN is received, the session is cleaned up and returns to
idle state. Alternatively, if the LCCE chooses to terminate the
session, it sends a CDN to the LAC, cleans up the session, and
moves the session to idle state.
7.4.2. OCRQ Recipient (LAC) States
State Event Action New State
----- ----- ------ ---------
idle Receive OCRQ, Send OCRP, wait-cs-answer
acceptable Place call
idle Receive OCRQ, Send CDN, idle
not acceptable clean up
idle Receive OCRP Send CDN, idle
clean up
idle Receive OCCN, Clean up idle
CDN
wait-cs-answer Call placement Send OCCN established
successful
wait-cs-answer Call placement Send CDN, idle
failed clean up
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RFC 3931 L2TPv3 March 2005
wait-cs-answer Receive OCRQ, Send CDN, idle
OCRP, OCCN clean up
established Receive OCRQ, Send CDN, idle
OCRP, OCCN clean up
wait-cs-answer, Receive CDN Clean up idle
established
wait-cs-answer, Local close Send CDN, idle
established request clean up
The states associated with the LAC for outgoing calls are as follows:
idle
If the OCRQ is received in error, respond with a CDN. Otherwise,
place the call, send an OCRP, and move to the wait-cs-answer
state.
wait-cs-answer
If the call is not completed or a timer expires while waiting for
the call to complete, send a CDN with the appropriate error
condition set, and go to idle state. If a circuit-switched
connection is established, send an OCCN indicating success, and go
to established state.
established
If the LAC receives a CDN from the peer, the call MUST be released
via appropriate mechanisms, and the session cleaned up. If the
call is disconnected because the circuit transitions to a
"disconnected" or "down" state, the LAC MUST send a CDN to the
peer and return to idle state.
7.5. Termination of a Control Connection
The termination of a control connection consists of either peer
issuing a StopCCN. The sender of this message SHOULD wait a full
control message retransmission cycle (e.g., 1 + 2 + 4 + 8 ...
seconds) for the acknowledgment of this message before releasing the
control information associated with the control connection. The
recipient of this message should send an acknowledgment of the
message to the peer, then release the associated control information.
When to release a control connection is an implementation issue and
is not specified in this document. A particular implementation may
use whatever policy is appropriate for determining when to release a
control connection. Some implementations may leave a control
connection open for a period of time or perhaps indefinitely after
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RFC 3931 L2TPv3 March 2005
the last session for that control connection is cleared. Others may
choose to disconnect the control connection immediately after the
last call on the control connection disconnects.
8. Security Considerations
This section addresses some of the security issues that L2TP
encounters in its operation.
8.1. Control Connection Endpoint and Message Security
If a shared secret (password) exists between two LCCEs, it may be
used to perform a mutual authentication between the two LCCEs, and
construct an authentication and integrity check of arriving L2TP
control messages. The mechanism provided by L2TPv3 is described in
Section 4.3 and in the definition of the Message Digest and Control
Message Authentication Nonce AVPs in Section 5.4.1.
This control message security mechanism provides for (1) mutual
endpoint authentication, and (2) individual control message integrity
and authenticity checking. Mutual endpoint authentication ensures
that an L2TPv3 control connection is only established between two
endpoints that are configured with the proper password. The
individual control message and integrity check guards against
accidental or intentional packet corruption (i.e., those caused by a
control message spoofing or man-in-the-middle attack).
The shared secret that is used for all control connection, control
message, and AVP security features defined in this document never
needs to be sent in the clear between L2TP tunnel endpoints.
8.2. Data Packet Spoofing
Packet spoofing for any type of Virtual Private Network (VPN)
protocol is of particular concern as insertion of carefully
constructed rogue packets into the VPN transit network could result
in a violation of VPN traffic separation, leaking data into a
customer VPN. This is complicated by the fact that it may be
particularly difficult for the operator of the VPN to even be aware
that it has become a point of transit into or between customer VPNs.
L2TPv3 provides traffic separation for its VPNs via a 32-bit Session
ID in the L2TPv3 data header. When present, the L2TPv3 Cookie
(described in Section 4.1), provides an additional check to ensure
that an arriving packet is intended for the identified session.
Thus, use of a Cookie with the Session ID provides an extra guarantee
that the Session ID lookup was performed properly and that the
Session ID itself was not corrupted in transit.
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In the presence of a blind packet spoofing attack, the Cookie may
also provide security against inadvertent leaking of frames into a
customer VPN. To illustrate the type of security that it is provided
in this case, consider comparing the validation of a 64-bit Cookie in
the L2TPv3 header to the admission of packets that match a given
source and destination IP address pair. Both the source and
destination IP address pair validation and Cookie validation consist
of a fast check on cleartext header information on all arriving
packets. However, since L2TPv3 uses its own value, it removes the
requirement for one to maintain a list of (potentially several)
permitted or denied IP addresses, and moreover, to guard knowledge of
the permitted IP addresses from hackers who may obtain and spoof
them. Further, it is far easier to change a compromised L2TPv3
Cookie than a compromised IP address," and a cryptographically random
[RFC1750] value is far less likely to be discovered by brute-force
attacks compared to an IP address.
For protection against brute-force, blind, insertion attacks, a 64-
bit Cookie MUST be used with all sessions. A 32-bit Cookie is
vulnerable to brute-force guessing at high packet rates, and as such,
should not be considered an effective barrier to blind insertion
attacks (though it is still useful as an additional verification of a
successful Session ID lookup). The Cookie provides no protection
against a sophisticated man-in-the-middle attacker who can sniff and
correlate captured data between nodes for use in a coordinated
attack.
The Assigned Cookie AVP is used to signal the value and size of the
Cookie that must be present in all data packets for a given session.
Each Assigned Cookie MUST be selected in a cryptographically random
manner [RFC1750] such that a series of Assigned Cookies does not
provide any indication of what a future Cookie will be.
The L2TPv3 Cookie must not be regarded as a substitute for security
such as that provided by IPsec when operating over an open or
untrusted network where packets may be sniffed, decoded, and
correlated for use in a coordinated attack. See Section 4.1.3 for
more information on running L2TP over IPsec.
9. Internationalization Considerations
The Host Name and Vendor Name AVPs are not internationalized. The
Vendor Name AVP, although intended to be human-readable, would seem
to fit in the category of "globally visible names" [RFC2277] and so
is represented in US-ASCII.
If (1) an LCCE does not signify a language preference by the
inclusion of a Preferred Language AVP (see Section 5.4.3) in the
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RFC 3931 L2TPv3 March 2005
SCCRQ or SCCRP, (2) the Preferred Language AVP is unrecognized, or
(3) the requested language is not supported by the peer LCCE, the
default language [RFC2277] MUST be used for all internationalized
strings sent by the peer.
10. IANA Considerations
This document defines a number of "magic" numbers to be maintained by
the IANA. This section explains the criteria used by the IANA to
assign additional numbers in each of these lists. The following
subsections describe the assignment policy for the namespaces defined
elsewhere in this document.
Sections 10.1 through 10.3 are requests for new values already
managed by IANA according to [RFC3438].
The remaining sections are for new registries that have been added to
the existing L2TP registry and are maintained by IANA accordingly.
10.1. Control Message Attribute Value Pairs (AVPs)
This number space is managed by IANA as per [RFC3438].
A summary of the new AVPs follows:
Control Message Attribute Value Pairs
Attribute
Type Description
--------- ------------------
58 Extended Vendor ID AVP
59 Message Digest
60 Router ID
61 Assigned Control Connection ID
62 Pseudowire Capabilities List
63 Local Session ID
64 Remote Session ID
65 Assigned Cookie
66 Remote End ID
68 Pseudowire Type
69 L2-Specific Sublayer
70 Data Sequencing
71 Circuit Status
72 Preferred Language
73 Control Message Authentication Nonce
74 Tx Connect Speed
75 Rx Connect Speed
Lau, et al. Standards Track [Page 80]

RFC 3931 L2TPv3 March 200510.2. Message Type AVP Values
This number space is managed by IANA as per [RFC3438]. There is one
new message type, defined in Section 3.1, that was allocated for this
specification:
Message Type AVP (Attribute Type 0) Values
------------------------------------------
Control Connection Management
20 (ACK) Explicit Acknowledgement
10.3. Result Code AVP Values
This number space is managed by IANA as per [RFC3438].
New Result Code values for the CDN message are defined in Section5.4. The following is a summary:
Result Code AVP (Attribute Type 1) Values
-----------------------------------------
General Error Codes
13 - Session not established due to losing
tie breaker (L2TPv3).
14 - Session not established due to unsupported
PW type (L2TPv3).
15 - Session not established, sequencing required
without valid L2-Specific Sublayer (L2TPv3).
16 - Finite state machine error or timeout.
Lau, et al. Standards Track [Page 81]

RFC 3931 L2TPv3 March 200510.6. Pseudowire Types
This is a new registry for IANA to maintain, there are no values
assigned within this document to maintain.
L2TPv3 Pseudowire Types
-----------------------
The Pseudowire Type (PW Type, see Section 5.4) is a 2-octet value
used in the Pseudowire Type AVP and Pseudowire Capabilities List AVP
defined in Section 5.4.3. 0 to 32767 are assignable by Expert Review
[RFC2434], while 32768 to 65535 are assigned by a First Come First
Served policy [RFC2434]. There are no specific pseudowire types
assigned within this document. Each pseudowire-specific document
must allocate its own PW types from IANA as necessary.
10.7. Circuit Status Bits
This is a new registry for IANA to maintain.
Circuit Status Bits
-------------------
The Circuit Status field is a 16-bit mask, with the two low order
bits assigned. Additional bits may be assigned by IETF Consensus
[RFC2434].
Bit 14 - New (N bit)
Bit 15 - Active (A bit)
Lau, et al. Standards Track [Page 83]

RFC 3931 L2TPv3 March 2005
[RFC3355] Singh, A., Turner, R., Tio, R., and Nanji, S., "Layer Two
Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5
(AAL5)", RFC 3355, August 2002.
[KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network
Security: Private Communications in a Public World",
Prentice Hall, March 1995, ISBN 0-13-061466-1.
[STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I: The
Protocols", Addison-Wesley Publishing Company, Inc., March
1996, ISBN 0-201-63346-9.
12. Acknowledgments
Many of the protocol constructs were originally defined in, and the
text of this document began with, RFC 2661, "L2TPv2". RFC 2661
authors are W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn and
B. Palter.
The basic concept for L2TP and many of its protocol constructs were
adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these
versions are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G.
Pall, W. Verthein, J. Taarud, W. Little, and G. Zorn.
Danny Mcpherson and Suhail Nanji published the first "L2TP Service
Type" version, which defined the use of L2TP for tunneling of various
L2 payload types (initially, Ethernet and Frame Relay).
The team for splitting RFC 2661 into this base document and the
companion PPP document consisted of Ignacio Goyret, Jed Lau, Bill
Palter, Mark Townsley, and Madhvi Verma. Skip Booth also provided
very helpful review and comment.
Some constructs of L2TPv3 were based in part on UTI (Universal
Transport Interface), which was originally conceived by Peter
Lothberg and Tony Bates.
Stewart Bryant and Simon Barber provided valuable input for the
L2TPv3 over IP header.
Juha Heinanen provided helpful review in the early stages of this
effort.
Jan Vilhuber, Scott Fluhrer, David McGrew, Scott Wainner, Skip Booth
and Maria Dos Santos contributed to the Control Message
Authentication Mechanism as well as general discussions of security.
Lau, et al. Standards Track [Page 87]

RFC 3931 L2TPv3 March 2005
James Carlson, Thomas Narten, Maria Dos Santos, Steven Bellovin, Ted
Hardie, and Pekka Savola provided very helpful review of the final
versions of text.
Russ Housley provided valuable review and comment on security,
particularly with respect to the Control Message Authentication
mechanism.
Pekka Savola contributed to proper alignment with IPv6 and inspired
much of Section 4.1.4 on fragmentation.
Aside of his original influence and co-authorship of RFC 2661, Glen
Zorn helped get all of the language and character references straight
in this document.
A number of people provided valuable input and effort for RFC 2661,
on which this document was based:
John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege,
Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and
review at the 43rd IETF in Orlando, FL, which led to improvement of
the overall readability and clarity of RFC 2661.
Thomas Narten provided a great deal of critical review and
formatting. He wrote the first version of the IANA Considerations
section.
Dory Leifer made valuable refinements to the protocol definition of
L2TP and contributed to the editing of early versions leading to RFC2661.
Steve Cobb and Evan Caves redesigned the state machine tables.
Barney Wolff provided a great deal of design input on the original
endpoint authentication mechanism.
Lau, et al. Standards Track [Page 88]

RFC 3931 L2TPv3 March 2005
Appendix A: Control Slow Start and Congestion Avoidance
Although each side has indicated the maximum size of its receive
window, it is recommended that a slow start and congestion avoidance
method be used to transmit control packets. The methods described
here are based upon the TCP congestion avoidance algorithm as
described in Section 21.6 of TCP/IP Illustrated, Volume I, by W.
Richard Stevens [STEVENS] (this algorithm is also described in
[RFC2581]).
Slow start and congestion avoidance make use of several variables.
The congestion window (CWND) defines the number of packets a sender
may send before waiting for an acknowledgment. The size of CWND
expands and contracts as described below. Note, however, that CWND
is never allowed to exceed the size of the advertised window obtained
from the Receive Window AVP. (In the text below, it is assumed any
increase will be limited by the Receive Window Size.) The variable
SSTHRESH determines when the sender switches from slow start to
congestion avoidance. Slow start is used while CWND is less than
SSHTRESH.
A sender starts out in the slow start phase. CWND is initialized to
one packet, and SSHTRESH is initialized to the advertised window
(obtained from the Receive Window AVP). The sender then transmits
one packet and waits for its acknowledgment (either explicit or
piggybacked). When the acknowledgment is received, the congestion
window is incremented from one to two. During slow start, CWND is
increased by one packet each time an ACK (explicit ACK message or
piggybacked) is received. Increasing CWND by one on each ACK has the
effect of doubling CWND with each round trip, resulting in an
exponential increase. When the value of CWND reaches SSHTRESH, the
slow start phase ends and the congestion avoidance phase begins.
During congestion avoidance, CWND expands more slowly. Specifically,
it increases by 1/CWND for every new ACK received. That is, CWND is
increased by one packet after CWND new ACKs have been received.
Window expansion during the congestion avoidance phase is effectively
linear, with CWND increasing by one packet each round trip.
When congestion occurs (indicated by the triggering of a
retransmission) one-half of the CWND is saved in SSTHRESH, and CWND
is set to one. The sender then reenters the slow start phase.
Lau, et al. Standards Track [Page 89]

RFC 3931 L2TPv3 March 2005
Appendix B: Control Message Examples
B.1: Lock-Step Control Connection Establishment
In this example, an LCCE establishes a control connection, with the
exchange involving each side alternating in sending messages. This
example shows the final acknowledgment explicitly sent within an ACK
message. An alternative would be to piggyback the acknowledgment
within a message sent as a reply to the ICRQ or OCRQ that will likely
follow from the side that initiated the control connection.
LCCE A LCCE B
------ ------
SCCRQ ->
Nr: 0, Ns: 0
<- SCCRP
Nr: 1, Ns: 0
SCCCN ->
Nr: 1, Ns: 1
<- ACK
Nr: 2, Ns: 1
B.2: Lost Packet with Retransmission
An existing control connection has a new session requested by LCCE A.
The ICRP is lost and must be retransmitted by LCCE B. Note that loss
of the ICRP has two effects: It not only keeps the upper level state
machine from progressing, but also keeps LCCE A from seeing a timely
lower level acknowledgment of its ICRQ.
LCCE A LCCE B
------ ------
ICRQ ->
Nr: 1, Ns: 2
(packet lost) <- ICRP
Nr: 3, Ns: 1
(pause; LCCE A's timer started first, so fires first)
ICRQ ->
Nr: 1, Ns: 2
(Realizing that it has already seen this packet,
LCCE B discards the packet and sends an ACK message)
<- ACK
Nr: 3, Ns: 2
Lau, et al. Standards Track [Page 90]

RFC 3931 L2TPv3 March 2005
(LCCE B's retransmit timer fires)
<- ICRP
Nr: 3, Ns: 1
ICCN ->
Nr: 2, Ns: 3
<- ACK
Nr: 4, Ns: 2
Appendix C: Processing Sequence Numbers
The Default L2-Specific Sublayer, defined in Section 4.6, provides a
24-bit field for sequencing of data packets within an L2TP session.
L2TP data packets are never retransmitted, so this sequence is used
only to detect packet order, duplicate packets, or lost packets.
The 24-bit Sequence Number field of the Default L2-Specific Sublayer
contains a packet sequence number for the associated session. Each
sequenced data packet that is sent must contain the sequence number,
incremented by one, of the previous sequenced packet sent on a given
L2TP session. Upon receipt, any packet with a sequence number equal
to or greater than the current expected packet (the last received
in-order packet plus one) should be considered "new" and accepted.
All other packets are considered "old" or "duplicate" and discarded.
Note that the 24-bit sequence number space includes zero as a valid
sequence number (as such, it may be implemented with a masked 32-bit
counter if desired). All new sessions MUST begin sending sequence
numbers at zero.
Larger or smaller sequence number fields are possible with L2TP if an
alternative format to the Default L2-Specific Sublayer defined in
this document is used. While 24 bits may be adequate in a number of
circumstances, a larger sequence number space will be less
susceptible to sequence number wrapping problems for very high
session data rates across long dropout periods. The sequence number
processing recommendations below should hold for any size sequence
number field.
When detecting whether a packet sequence number is "greater" or
"less" than a given sequence number value, wrapping of the sequence
number must be considered. This is typically accomplished by keeping
a window of sequence numbers beyond the current expected sequence
number for determination of whether a packet is "new" or not. The
window may be sized based on the link speed and sequence number space
and SHOULD be configurable with a default equal to one half the size
of the available number space (e.g., 2^(n-1), where n is the number
of bits available in the sequence number).
Lau, et al. Standards Track [Page 91]

RFC 3931 L2TPv3 March 2005
Upon receipt, packets that exactly match the expected sequence number
are processed immediately and the next expected sequence number
incremented. Packets that fall within the window for new packets may
either be processed immediately and the next expected sequence number
updated to one plus that received in the new packet, or held for a
very short period of time in hopes of receiving the missing
packet(s). This "very short period" should be configurable, with a
default corresponding to a time lapse that is at least an order of
magnitude less than the retransmission timeout periods of higher
layer protocols such as TCP.
For typical transient packet mis-orderings, dropping out-of-order
packets alone should suffice and generally requires far less
resources than actively reordering packets within L2TP. An exception
is a case in which a pair of packet fragments are persistently
retransmitted and sent out-of-order. For example, if an IP packet
has been fragmented into a very small packet followed by a very large
packet before being tunneled by L2TP, it is possible (though
admittedly wrong) that the two resulting L2TP packets may be
consistently mis-ordered by the PSN in transit between L2TP nodes.
If sequence numbers were being enforced at the receiving node without
any buffering of out-of-order packets, then the fragmented IP packet
may never reach its destination. It may be worth noting here that
this condition is true for any tunneling mechanism of IP packets that
includes sequence number checking on receipt (i.e., GRE [RFC2890]).
Utilization of a Data Sequencing Level (see Section 5.4.3) of 1 (only
non-IP data packets require sequencing) allows IP data packets being
tunneled by L2TP to not utilize sequence numbers, while utilizing
sequence numbers and enforcing packet order for any remaining non-IP
data packets. Depending on the requirements of the link layer being
tunneled and the network data traversing the data link, this is
sufficient in many cases to enforce packet order on frames that
require it (such as end-to-end data link control messages), while not
on IP packets that are known to be resilient to packet reordering.
If a large number of packets (i.e., more than one new packet window)
are dropped due to an extended outage or loss of sequence number
state on one side of the connection (perhaps as part of a forwarding
plane reset or failover to a standby node), it is possible that a
large number of packets will be sent in-order, but be wrongly
detected by the peer as out-of-order. This can be generally
characterized for a window size, w, sequence number space, s, and
number of packets lost in transit between L2TP endpoints, p, as
follows:
Lau, et al. Standards Track [Page 92]

RFC 3931 L2TPv3 March 2005
If s > p > w, then an additional (s - p) packets that were otherwise
received in-order, will be incorrectly classified as out-of-order and
dropped. Thus, for a sequence number space, s = 128, window size, w
= 64, and number of lost packets, p = 70; 128 - 70 = 58 additional
packets would be dropped after the outage until the sequence number
wrapped back to the current expected next sequence number.
To mitigate this additional packet loss, one MUST inspect the
sequence numbers of packets dropped due to being classified as "old"
and reset the expected sequence number accordingly. This may be
accomplished by counting the number of "old" packets dropped that
were in sequence among themselves and, upon reaching a threshold,
resetting the next expected sequence number to that seen in the
arriving data packets. Packet timestamps may also be used as an
indicator to reset the expected sequence number by detecting a period
of time over which "old" packets have been received in-sequence. The
ideal thresholds will vary depending on link speed, sequence number
space, and link tolerance to out-of-order packets, and MUST be
configurable.
Editors' Addresses
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
EMail: jedlau@cisco.com
W. Mark Townsley
cisco Systems
EMail: mark@townsley.net
Ignacio Goyret
Lucent Technologies
EMail: igoyret@lucent.com
Lau, et al. Standards Track [Page 93]

RFC 3931 L2TPv3 March 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Lau, et al. Standards Track [Page 94]